Partition Magic is the most popular program for resizing partitons, and for good reason.  It is capable of non-destructive partition resizing, meaning that you can increase the size of the partition that WinXP is on without loosing data.  Of course, I recommend you have a recent back-up, since the operation is not 100% fool-proof.

There are a few other programs out there, but PM is the best around.

If you have the space, I would give WinXP about 15 GB.  That will allow plenty of space for the OS and any programs and tools you choose to install.  Disk space is so cheap now that it doesn't make sense to paint yourself into corners anymore.