Can you ping a different host on the corporate network? (if yes, it might be a firewal on your XP machine causing the problem)
Does your ip address (if you go to, for example,
www.ipchicken.com) change when you connect to your VPN? (If not, it's either a split tunnel vpn (where some traffic goes through it and other traffic doesn't, or it's not working)
Can you normally ping your XP computer if you're plugged into the physical network? (If you can, it might be a firewall on the VPN, if you can't it might be a firewall on your XP computer)
My advice is to talk to tech folks at your work.. there are a ton of configurations and stuff that a VPN can have, and any help I can give you is purely guesswork. Hopefully there's at least one tech person at work who can step through troubleshooting with you.