Vista - Explorer.exe using 50% CPU Usage...

Hello everyone. I've had this PC for about 10 months and it has been running great up until a few weeks ago. I don't know if it is a hardware/driver conflict, a virus, or what exactly, but Explorer.exe is always running at 50% CPU usage. Here are my basic system specs:

Windows Vista Home Premium (came pre-installed on my HP computer) with SP 1
AMD Athlon X2 BE-2400 (I upgraded this from an Athlon X2 3800+ a month or two ago)
2 GB DDR2 Ram (I upgraded this as well a few months back from 1 GB)
Integrated NVIDIA graphics

I have this exact same PC in a different room (minus the CPU and RAM upgrade), and it runs just fine... so I dunno what the deal is with this one. Anyhow, I've done some research on this problem and nothing works. I've tried creating a new account, but immediately Explorer.exe spiked up to 50% on it. I tried running the PC in safe mode, and the same thing happened as well. I've run the PC through Adaware, AVG, and a few other anti-everything programs. Does anyone have any ideas on what could be going on, or should I just format this thing?