Daniel Iversen posted a guide about making Windows 2000 run (rather well) on only 32MB RAM

I had an old but very leight-weight small subnotebook that I needed for after hours support for one of my clients... I needed to install Windows 2000 to use my mobile phone as a modem to access the high speed 3G data network. Your reason might be that you have an old PC you don't want to throw away and Windows 95/98/Me might not give you the features you are looking for... be aware though that if Windows 98 meets your needs it is stupid to upgrade because you have lots less memory to work with when installing Windows 2000.

Making Windows 2000 run (rather well) on only 32MB RAM