20GB? Remember when Windows came on 6 floppy disks? No matter how fast your computer runs, Windows will slow it down to a crawl.
Meanwhile, I run Kubuntu 8.04 on a 2000 - 2001 vintage IBM ThinkPad. 800 MHz Pentium-III, 192 MB of memory, and 20 GB of disk. Yes, it is a little slow, but it is usable. It was originally a Win98 machine. It has been host to several Linux distros. I don't think XP would even install on it. Vista, not even funny. Latest version of Ubuntu/Kubuntu? 30 min and it's up and running...