Random Massive Performance Drop

So I was just going to reformat, but I thought I'd check up with the knowledgeable experts over here.

This isn't like something I've ever seen before. It's strange to say the least.

Basically, about 24 hours ago I turned my computer on to have it perform abnormally slow. It took a little longer than usual to boot up, and once it got past the Windows XP loading bar, it took about a minute just to load the login page. To put that into perspective, it spent 10 seconds at the loading bar and 60 trying to load the login page.

Anyway, it took about a good 30 seconds for the desktop to show up.

But here's where things get weird. It's not like normal dragging performance where everything is slow. I could instantly close all of my programs running in the sidebar without any lag at all.

But while I was doing this, it took about 5 minutes for Live Messenger, a program that starts up with the login, to open.

While it was running in the background, failing to open, I could browse Chrome at normal speed.

My Computer will open and load in about 1 second right as soon as the desktop appears too.

Then, once everything was all done starting up and such, things got interesting. I can use one program with relative ease and no lag, but when switching from one to the other, say Chrome to MSN, all hell breaks loose. Things just lock right up for a while.

This thing was performing beautifully as of 2 days ago.

The only thing I notice is now my harddrive (Or something in there) wont shut up with the loading noise. You know the sort of rumbling they make when they're loading something (The old ones anyway?). Mine never used to do this, but it'll rumble for a second, go quiet for 2 seconds or so, and then repeat. It never stops doing this even when nothing is loading and such.

I'm considering a reformat, but if it's a hardware issue a reformat wont do me much good, huh? I'm just wondering why it'd happen out of the blue like this, since the symptoms aren't like one of a virus at all.

Thanks again, guys. Your help is much appreciated.