When I run single player and try to start/load a game I get this error:
JK2: v1.02a win-x86 Mar 4 2002
Initialising zone memory .....
----- FS_Startup -----
Current search path:
C:\Program Files\LucasArts\Star Wars JK II Jedi Outcast\GameData\base\Ultimate_Maul.pk3 (26 files)
C:\Program Files\LucasArts\Star Wars JK II Jedi Outcast\GameData\base\NeCrO.pk3 (20 files)
C:\Program Files\LucasArts\Star Wars JK II Jedi Outcast\GameData\base\assets2.pk3 (289 files)
C:\Program Files\LucasArts\Star Wars JK II Jedi Outcast\GameData\base\assets1.pk3 (8011 files)
C:\Program Files\LucasArts\Star Wars JK II Jedi Outcast\GameData\base\assets0.pk3 (6674 files)
C:\Program Files\LucasArts\Star Wars JK II Jedi Outcast\GameData/base

----------------------
15020 files in pk3 files
execing default.cfg
couldn't exec jk2config.cfg
couldn't exec autoexec.cfg
...detecting CPU, found Intel Pentium IV

------- Input Initialization -------
Skipping check for DirectInput
Joystick is not active.
------------------------------------

------- Force Feedback Initialization -------
...inhibited, not initializing

----- Client Initialization -----
----- Initializing Renderer ----
-------------------------------
----- Client Initialization Complete -----
--- Common Initialization Complete ---
Working directory: C:\Program Files\LucasArts\Star Wars JK II Jedi Outcast\GameData
----- R_Init -----
Initializing OpenGL subsystem
...initializing QGL
...calling LoadLibrary( 'C:\WINDOWS\System32\opengl32.dll' ): succeeded
...setting mode 3: 640 480 FS
...using desktop display depth of 32
...calling CDS: ok
...registered window class
...created window@0,0 (640x480)
Initializing OpenGL driver
...getting DC: succeeded
...GLW_ChoosePFD( 32, 24, 8 )
...50 PFDs found
...hardware acceleration found
...PIXELFORMAT 8 selected
...creating GL context: succeeded
...making context current: succeeded
Initializing OpenGL extensions
...GL_S3_s3tc available
...GL_EXT_texture_compression_s3tc available
...no tc preference specified
.....using GL_EXT_texture_compression_s3tc
...using GL_EXT_texture_env_add
...GL_EXT_texture_filter_anisotropic available
...using GL_EXT_texture_filter_anisotropic
...Using GL_EXT_texture_edge_clamp
...using WGL_EXT_swap_control
...using GL_ARB_multitexture
...using GL_EXT_compiled_vertex_array
...using GL_EXT_point_parameters
Initializing OpenGL extensions
...GL_S3_s3tc available
...GL_EXT_texture_compression_s3tc available
...no tc preference specified
.....using GL_EXT_texture_compression_s3tc
...using GL_EXT_texture_env_add
...GL_EXT_texture_filter_anisotropic available
...using GL_EXT_texture_filter_anisotropic
...Using GL_EXT_texture_edge_clamp
...using WGL_EXT_swap_control
...using GL_ARB_multitexture
...using GL_EXT_compiled_vertex_array
...using GL_EXT_point_parameters
WARNING: suspicious gamma tables, using linear ramp for restoration

GL_VENDOR: NVIDIA Corporation
GL_RENDERER: GeForce2 MX/AGP/SSE2
GL_VERSION: 1.2.2
GL_EXTENSIONS: GL_ARB_imaging GL_ARB_multitexture GL_ARB_texture_compression GL_ARB_texture_cube_map GL_ARB_texture_env_add GL_ARB_texture_env_combine GL_ARB_texture_env_dot3 GL_ARB_transpose_matrix GL_S3_s3tc GL_EXT_abgr GL_EXT_bgra GL_EXT_blend_color GL_EXT_blend_minmax GL_EXT_blend_subtract GL_EXT_compiled_vertex_array GL_EXT_draw_range_elements GL_EXT_fog_coord GL_EXT_packed_pixels GL_EXT_paletted_texture GL_EXT_point_parameters GL_EXT_rescale_normal GL_EXT_secondary_color GL_EXT_separate_specular_color GL_EXT_shared_texture_palette GL_EXT_stencil_wrap GL_EXT_texture_compression_s3tc GL_EXT_texture_edge_clamp GL_EXT_texture_env_add GL_EXT_texture_env_combine GL_EXT_texture_env_dot3 GL_EXT_texture_cube_map GL_EXT_texture_filter_anisotropic GL_EXT_texture_lod GL_EXT_texture_lod_bias GL_EXT_texture_object GL_EXT_vertex_array GL_EXT_vertex_weighting GL_IBM_texture_mirrored_repeat GL_KTX_buffer_region GL_NV_blend_square GL_NV_evaluators GL_NV_fence GL_NV_fog_distance GL_NV_light_max_exponent GL_NV_packed_depth_stencil GL_NV_register_combiners GL_NV_texgen_emboss GL_NV_texgen_reflection GL_NV_texture_env_combine4 GL_NV_texture_rectangle GL_NV_vertex_array_range GL_NV_vertex_array_range2 GL_NV_vertex_program GL_SGIS_generate_mipmap GL_SGIS_multitexture GL_SGIS_texture_lod GL_WIN_swap_hint WGL_EXT_swap_control
GL_MAX_TEXTURE_SIZE: 2048
GL_MAX_ACTIVE_TEXTURES_ARB: 2

PIXELFORMAT: color(32-bits) Z(24-bit) stencil(8-bits)
MODE: 3, 640 x 480 fullscreen hz:60
GAMMA: hardware w/ 0 overbright bits
CPU: Intel Pentium IV
rendering primitives: single glDrawElements
texturemode: GL_LINEAR_MIPMAP_LINEAR
picmip: 1
texture bits: 0
lightmap texture bits: 0
multitexture: enabled
compiled vertex arrays: enabled
texenv add: enabled
compressed textures: enabled
compressed lightmaps: disabled
texture compression method: GL_EXT_texture_compression_s3tc
anisotropic filtering: enabled
Initializing Shaders
----- finished R_Init -----

------- Input Initialization -------
Skipping check for DirectInput
Joystick is not active.
------------------------------------

------- Force Feedback Initialization -------
...inhibited, not initializing


------- sound initialization -------
Initializing DirectSound
- using ancient version of DirectX -- this will slow FPS
locked hardware. ok
----- Sound Info -----
sound system is muted
1 stereo
32768 samples
16 samplebits
1 submission_chunk
22050 speed
0xc938060 dma buffer
No background file.
----------------------
------------------------------------

--- ambient sound initialization ---
Sound memory manager started
UI menu load time = 4558 milli seconds
------ Server Initialization ------
JK2: v1.02a win-x86 Mar 4 2002
Server: kejim_post
RE_Shutdown( 0 )
Initializing Shaders
----- CL_Shutdown -----
RE_Shutdown( 1 )
Shutting down OpenGL subsystem
...wglMakeCurrent( NULL, NULL ): success
...deleting GL context: success
...releasing DC: success
...destroying window
...resetting display
...shutting down QGL
...unloading OpenGL DLL
-----------------------
----- Server Shutdown -----
---------------------------
bad parameter in external weapon data 'firingforce'


Everything was fine until I decided to reinstall to get rid of the new patch... then this starts happening, I've already reinstalled once more.... what could be the problem?

Seems obvious, but ill ask anyhow.... when you say you reinstalled, did you UN-install first (e.g. delete the entire game folder?)