Sounds, Folders, Right Click

I've literally just started having problems with my computer. Sporadically I have lost the ability to right-click, sometimes whenever I do so on either a folder or on an application, no little menu comes up

Also, when browsing through folders, the loading starts to become very slow and sometimes the blue menu at the side of the window isn't there at all. When I click on images to preview, or folders to open, sometimes nothing happens at all and the window just remains the same.

ALSO whilst trying to open some applications I get the error pop-up sound, but no error message box comes up to tell me what the error is. It just makes the noise, and I close the half-loaded application without it crashing (This happened with Photoshop, Adaware, Outlook Express, and once with Sonic RecordNow - all started loading, the error noise was heard, and the program left half loaded or missing parts) I've tried again and it seems it's all hit and miss like the other symptoms...sometimes i'll be able to right-click normally, sometimes i'll be able to browse files normally, sometimes programs will load, but the problem still (at random moments, it seems) arises again.

When Alt Ctrl Delete -ing, either nothing comes up and an error noise is heard, or the little window with the information does come up, but the desktop behind it flickers as if constantly reloading very fast.

The only error sound in which a box popped up, was one telling me "xvidcore.dll was not found". I clicked OK and everything reloaded as if windows explorer had crashed (the start bar, desktop etc).

I haven't done anything as of now, too scared to restart, or try and open any applications it might not like. I can't be exactly sure when I started to have problems, as I leave the computer on for days at a time sometimes (it's been running for about four days this time, about three days is normally the maximum) because I use BitTorrent a lot seeding and downloading.