If you installed mods then it the mods causing your game to crash (because of not loading correctly, too many/little pixles, not integrated in the gta3.img file correctly, not working because the mod sucks, etc)
Depending on the mod and which files have been changed determines how easy it is to reverse this.
If the gta3.img file is the only file edited then try and get a clean file and replace your gta3.img. However this is not a guarantee this will work, because gta:sa and file modification is unpredictable at best. If other files were modded then replace those with clean files. Otherwise reinstall gta:sa and samp.
A note for future:
Using vista ensure "read protection" is not applicable on the files/folders your modding, otherwise it causes a crash.
Copy any files your are modding, or just copy gta:sa (i usually copy the whole thing just in case)
Only install one mod at a time then test it. If you install 5mods and they are ok, and number 6 =crash then you only have to un-install mod number 6, not all 6 mods.
Other than that, there's not much else to say. Hopefully you will get the problem fixed and play online again. alsoi have modded gta3, vc, sa, 4, and other games, and i have made the exact same mistakes, which i why i back up entire game folders now, so i have original game files/folders. Games (esp gta) are unpredictable with mods (esp 4!) so its worth it to have a back up just in-case, rather then uninstalling the whole game and re-installing it over and over because of one mod which does not work
