I know I'm answering a bit late, but I think there's a possibility that it could be your memory. Have you got any spare RAM you can try?
Also, for speccing a new PC, what is your budget range?
EDIT: Nevermind, here's a spec for you, though if you have an agp gfx card you may need to buy yourself a pci express one as well as a CPU/Motherboard/Memory.
Asus P5Q Intel P45 (Socket 775) PCI-Express DDR2 Motherboard £103.49
Intel Pentium Dual-Core E5200 "LGA775 Core 2" 2.50GHz (800FSB) - Retail £68.99
OCZ Platinum Revision 2 XTC 2GB (2x1GB) PC2-6400C4 Dual Channel Series DDR2 (OCZ2P800R22GK) £19.99
Total : £203.39 inc shipping (which you'll get free anyway so -£9.50)
That system should last you a while. A fair graphics card should be a Asus ATI Radeon HD 4670 512MB GDDR3 TV-Out/Dual DVI/HDMI (PCI-Express) for £60.94 inc VAT. There are cheaper ones, such as a Geforce 8400 for £27 or an ATI 3450 for around £30, but I'm not quite sure how good these are for your uses, though someone else might....
Hope this helps