C4U, have you made your choice? I looked at the MSI FX420x that GH recommended its a very nice rig. Specialy with 12 months to pay. I'm sure if you pay cash or straight credit card transaction, you can get it cheaper. Just get a licensed 64 bit Win7 OS and your set. The shop will install it for you, free na labor. Download Microsoft Security Essentials and you will be relatively safe from viruses.

BTW, slowing down of computers can also be due to the fragmentation of your hdd or an over loaded registry.