Computer runs TOO SLOW!

I have a Dell XPS M1530. I've had it about a year and a half, but about a month and a half ago it started running very slow. When I say slow I mean it takes around 5-10 minutes just to load the desktop when I turn it on (It used to take about 1-2). Sometimes the screen turns black for almost 5 minutes after typing in my computer password. When I click on Internet Explorer or Firefox I have to wait another 3-5 minutes before one of them finally loads, and after they load it takes another minute or two for me to load a website other than Google. Besides the internet it is very choppy when I do things like click on My Documents, My Computer etc. It holds 500G of space and has about half still free. I am running it on Vista Home edition SP2, 64-bit operating system.

Around the time it slowed down I do remember a Windows Vista Recovery malware of some sort got on my computer. I looked up how to get rid of it and it worked, after which I immediately installed BitDefender. I am pretty sure that it was around that time that it started slowing down, maybe the malware did something to it?

I ran BitDefender numerous times but no viruses or malware comes up. However, I do notice that my CPU is almost always at 80-100%. I don't know why this is and I don't want to end something I shouldn't. I adjusted the preferences on all of my software so it doesn't load automatically, but even when I'm only using one window in Firefox the CPU is still at 100%. I tried cleaning out the fan but this didn't work either. Any ideas would be really helpful, also let me know if you need any more info.