If you want to run Ubuntu and XP as virtual machines I believe you'll have to use VMWare instead of VPC. If memory serves me, VPC will only run various Wiindows OS's.
On my Gateway laptop I've got XP as the host OS with another XP as a client just to run my Delorme program. I'm using Ubuntu for surfing the web which eliminates any problems with viruses or drive-by malware installs.
VMWare offers a VMWare Player and VMWare server free of charge and the Server will get you going for the XP which you'll need to install. One advantage to VMWare is you can download pre-installed linux "appliances" off the web and just unzip them and fire them up, no installation needed. The free VMWare player will run these appliances. Another advantage to this type of setup is that creating a backup copy of the client OS's is just a copy and paste operation. You don't need any special software. I just store a copy of my XP/Delorme OS in another folder and when something goes wrong it takes about 5 minutes to copy and paste a new copy and I'm back in business.
A Google search for "vmware images" will get you several sites that offer the pre-installed linux "appliances" free.
http://www.vmware.com/appliances/directory/
http://vmplanet.net/
Have fun!