Using My Mac as a webserver for my VMware VM
Have you tried 'machinename.local?' (eg mymac.local)

I have a similar setup (and many many variations thereof) and usually the Windows VM can see the Mac via the address 'millie.local', where millie is the machine name.

If you go and look at the 'Sharing' tab of the System Preferences program, you should see what '.local' name your machine will appear as.

This doesn't work so well with Server 2003 (and presumably Server 2008) as .local has a different meaning with Windows AD networks, but it ought to be fine with XP.