I've just seen a clean Windows XP Home Edition installation crawling on a Celeron with 666 MHz and 128 MB of RAM (and sufficient video card and HD). There were no software installed, just SP3, all Windows Updates, and Avast Free Antivirus. It was impossible to install anything else, the memory usage was about 250 MB of virtual memory, any attempt took ages and the installer process dies silently thereafter.
The installation was brand new, no viruses, no polluted registry, no other problem. The antivirus took under 20 MB. According to Microsoft, the system requirements are P233 (P300 recommended) and 64 MB with 128 MB recommended. So are they just lying or what is to blame?