Well, it really depends on what you're doing. I assume that WinXP is your main machine, in which case I would probably split it 60/20 in favour of XP.
For the 20Gb you leave for Linux I would advocate at least 4 partitions.
/boot - 100 to 150Mb
/tmp - anything from 500Mb - 1Gb
swap - Some, probably 512Mb or similar will do.
/ - The rest of the space.
I always have a /tmp partition, as I have had bad experiences of badly behaved apps filling up /tmp and as a consequence of not having a separate partition, filling up the entire disk...
You probably don't need a separate /home partition, unless of course you are often reinstalling the OS and don't want to nuke your own files
Whatever you do, it will still quite flexible afterwards. You can also add a new disk, create a partition, and mount it as /var if the existing /var is running out of space.