100% CPU Usage?

Starting today whenever I open more than a few programs and work on my Dell for a little while my computer all of a sudden dramatically slows down.  So I pressed Ctrl+Alt+Del to check my performance in the Task Manager and I noticed that my CPU usage is at a stable 100% and it doesn't move from there! I tried restarting the computer to get it back down to normal but it only fixes the problem temporarily. Once i open a few programs up again (ie. AOL, IE, AIM, Yahoo! Messanger, Outlook Express, VB.net, Service Manager, SQL 2000) it works fine for a little while and then it just stabilizes at 100% again. This didn't start until today.

I can usually run these prgrams with no trouble at all and still keep a low CPU usage. I don't know what happened I didn't change a thing.  I also pretty much have everything unchecked on MSCONFIG except for 2 or 3 manadatory items.

What can I do? I know having my CPU usage at 100% for a long time is bad for my Dell.

Thanks!

Dell Dimension 4100 Series
Pentium III 933 Mhz
384MB SDRAM at 133 MHz
40GB Ultra ATA/100 (7200RPM) hard drive
16X NEC DV-5800A DVD-ROM w/ software decoding
TDK VeloCD Atapi/E-IDE CD ReWriter 48x/24x/48x
V.90/56K Conexant Speakerphone PCI Telephony Modem
Creative SB Live! Value Digital
32MB NVidia TNT2 M64 4X AGP video card
HP Deskjet 932C printer
Samsung SyncMaster 950p 19" CRT Monitor
Monsoon Multimedia MH-500 speakers
Quietkey Keyboard Standard
Microsoft Intellimouse
Windows XP Pro SP1
Office XP Pro
Norton Antivirus 2003 Pro