What the hell is wrong with my computer?

[Esuohlim]

For the last two or three months now, my computer has been acting up.

It freezes sometimes when you try to open more than one program at a time. It goes so goddamn slow that it sometimes can't even keep up with typing, so when I type something like "Sample phrase" it'll appear like "Smpl phse".

Also, I'm getting a considerable amount of Explorer "Illegal Operations" which either freeze my computer or give me the blue error screen where you need to Ctrl-Alt-Delete.

So, what seems to be the problem? My computer never used to be like this.