Lol guys, thanks all for the replies, and Shitman, you can go f yourself because now I can enter the server, BUT the problem is that every time a vehicle explodes near me I crash.
Summary:
I tried running both (samp.exe and gta_sa.exe) as administrator, still crashing.
I did delete the folder 'GTA San Andreas User Files' in Documents, reinstalled, yet same result.
I also tried setting it with compatibility mode for both Windows XP SP3 and Vista SP1, but this is what makes the game crash on the loading screen, so I disabled them.
Also, I tried uninstalling my antivirus (it was a trial anyway, was going to expire in 8 days), but that didn't work either.
Now, MidNight's reply sounds interesting because this started to happen all of a sudden since like 1 month ago, maybe something's messed up in the registry or there's a problem with the graphics card drivers. I'll think about what the problem could be, then maybe try the tool you mentioned. Not gonna do it now because I don't have much experience with the registry and don't wanna make things worse.
Anyway thanks everyone but FiNaL, will post if I happen to find the solution