I built my last computer while still in college. Since I was still in school for computer science I needed a computer that would run several software compilers quickly, I was also a gamer so I needed a powerful video card.
I purchased a barebones system (case and motherboard) from a company on Pricewatch.com. I then ordered RAM and a new Gforce video card for the system. Since I already had a Dell desktop I transferred the CD Drives and Hard Drives to the new systems since they were still good.
Once I had all the parts it took about an hour to assemble the system. I tried to be tricky and not reinstall windows on the Hard Drive. At first this did not work at all. I booted the system and windows would not load at all. I then put the Windows CD in the drive and ran a Windows repair. Presto the system booted fine and I did not have to reinstall windows.
I built this system 8 years ago and it is still running strong. If you can always build a system, it is cheaper and will generally last much longer.