Can't turn off properly, Can't restart, please help!

Hi, I'm currently using windows xp proffesional 64 bit edition, and it's not allowing me to restart, turn off my computer, or open barely any programs. Only programs that are working currently are ones that open on startup, and I dunno why, but internet explorer is the only other thing that works as well. Whenever I try to open a program, in the task manager it shows 700 kb for memory, and nothing happens. I am currently doing a full scan with avast! but the scan rate dropped when it reached some .temp folder. Please help asap! I can't do anything on this computer as of now.