PPTP should still be secure as the data is encrypted. Problems were found in the past version of Windows and were already patched. Windows XP should have this problem covered already and probably applied other security features on top of that. For more information on this, read the following:
http://www.microsoft.com/technet/network/vpn/vpnfaq.mspx
Newer version of Windows will have SSTP that will avoid other VPN access issues. You might be interested in the following read:
http://technet.microsoft.com/en-us/magazine/2007.06.cableguy.aspx