Use it to run VM's of various Operating Systems and browser combinations. Various Internet Explorer versions (6/7/8) can be useful for customers insisting it look good on machines of this vintage. Older Firefox and Safari will run native on XP. Test users can 'Remote Desktop' (Terminal Server) into the box from remote locations - they don't have to sit in front of it.
XP also has the ability to IIS aka Internet Information Server so it can be used as a modest sided development environment for Windows hosted websites. Handy to throw up a temporary sub-domain that can easily handle ~10k hits per hour.
The IIS install will also include a small SMTP email server (MTA - no mailboxes) that can be handy for testing scripts that don't queue or retry sending.