Open notepad and copy the following text into the document:
mat_antialias 0
mat_forceaniso 0
mat_hdr_enabled 0
mat_hdr_level 0
mat_picmip 2
mat_trilinear 0
mat_vsync 0
r_rootlod 2
mat_dxlevel 81 //Changes DX level to 8.1, remove this if you'd rather have your default
mat_monitorgamma "1.6" //Ingame Gamma Settings
//FPS Settings
fps_max 0 //Setting to 0 sets no max limit
cl_detaildist "0"
cl_detailfade "0"
cl_drawmonitors "0"
cl_ejectbrass "0"
cl_forcepreload "1"
cl_lagcomp_errorcheck "1"
cl_lagcompensation "1"
cl_downloadfilter "nosounds"
cl_ejectbrass "0"
cl_forcepreload 1
cl_phys_props_enable "0"
cl_phys_props_max "0"
cl_show_splashes "0"
cl_smoothtime "0.01"
cl_smooth 1
commentary 0
mat_aaquality "0"
mat_autoexposure_max "0"
mat_autoexposure_min "0"
mat_bloomscale "0"
mat_bufferprimitives "0"
mat_bumpmap "0"
mat_clipz "1"
mat_compressedtextures "1"
mat_diffuse "1"
mat_disable_bloom "1"
mat_disable_fancy_blending "1"
mat_disable_lightwarp "1"
mat_disable_ps_patch "1"
mat_envmapsize "0"
mat_excludetextures "1"
mat_envmaptgasize "0"
mat_fastspecular "1"
mat_filterlightmaps "1"
mat_filtertextures "1"
mat_forceaniso "0"
mat_forcehardwaresync "0"
mat_forcemanagedtextureintohardware "0"
mat_framebuffercopyoverlaysize "0"
mat_hdr_enabled "0"
mat_hdr_level "0"
mat_hdr_manual_tonemap_rate "0"
mat_mipmaptextures "0"
mat_lightmap_pfms "0"
mat_maxframelatency "0"
mat_max_worldmesh_vertices "0"
mat_parallaxmap "0"
mat_picmip "2"
mat_queue_mode "-1"
mat_reducefillrate "1"
mat_shadowstate "0"
mat_show_ab_hdr "0"
mat_showlightmappage "-1"
mat_specular "0"
mat_texture_limit "-1"
mat_trilinear "0"
mat_use_compressed_hdr_textures "1"
mat_showenvmapmask "0"
mat_showlowresimage "0"
mat_showmaterials "0"
mat_showmaterialsverbose "0"
mat_supportflashlight "0"
mat_wateroverlaysize "0"
mat_motion_blur_enabled "0"
mat_motion_blur_percent_of_screen_max "0"
mat_softwarelighting "0"
mp_decals "0"
muzzleflash_light "0"
net_maxfragments "1280"
net_maxfragments "1280"
net_showevents "0"
npc_height_adjust "1"
props_break_max_pieces "0"
props_break_max_pieces_perframe "0"
r_3dnow "1"
r_3dsky "0"
r_PhysPropStaticLighting "0"
r_WaterDrawReflection "0"
r_WaterDrawRefraction "0"
r_ambientboost "0"
r_cheapwaterend "1"
r_cheapwaterstart "1"
r_decal_cullsize "0"
r_decals "0"
r_dopixelvisibility "0"
r_drawbatchdecals "0"
r_drawflecks "0"
r_drawmodeldecals "0"
r_drawmodelstatsoverlaymax "1.5"
r_drawmodelstatsoverlaymin "0.01"
r_drawspecificstaticprop "0"
r_dynamic "0"
r_eyeglintlodpixels "0"
r_eyemove "0"
r_eyes "0"
r_eyeshift_x "0"
r_eyeshift_y "0"
r_eyeshift_z "0"
r_eyesize "0"
r_fastzreject "0"
r_flashlightrendermodels "0"
r_unloadlightmaps "1"
r_flashlightrenderworld "0"
r_flex "0"
r_forcewaterleaf "0"
r_lightaverage "0"
r_lod "2"
r_staticprop_lod "4"
r_maxdlights "0"
r_maxmodeldecal "0"
r_maxnewsamples "0"
r_maxsampledist "0"
r_minnewsamples "0"
r_mmx "1"
r_norefresh "0"
r_occlusion "0"
r_renderoverlayfragment "0"
r_queued_decals "1"
r_rootlod "2"
r_ropetranslucent "0"
r_shadowmaxrendered "32"
r_shadowrendertotexture "1"
r_shadows "0"
r_spray_lifetime "0.1"
r_sse "1"
r_sse2 "1"
r_teeth "0"
r_staticpropinfo "0"
r_updaterefracttexture "0"
r_updaterefracttexture "0"
r_visualizeproplightcaching "1"
r_waterdrawreflection "0"
r_waterforceexpensive "0"
r_waterforcereflectentities"0"
rope_averagelight "0"
rope_collide "0"
rope_shake "0"
rope_smooth "0"
rope_smooth_enlarge "0"
rope_smooth_maxalpha "0"
rope_smooth_maxalphawidth "0"
rope_smooth_minalpha "0"
rope_smooth_minwidth "0"
rope_subdiv "0"
rope_wind_dist "0"
//Blood settings
violence_ablood 1
violence_agibs 1
violence_hblood 1
violence_hgibs 1
Save as 'all files' into the right folder (steam -> steamapps -> *name* -> Team Fortress 2 -> tf -> cfg) with the name autoexec.cfg.
You will then want to open steam go to the My Games tab, right click Team Fortress 2 and select properties. Click the Set Launch Options and put in:
-dxlevel 81
This will turn off all the pretty stuff in the game but will at least hopefully allow you to play the game with your low end video card.