Since a lot of people here are using PPTP VPNs, I thought I'd pass this on.
http://www.microsoft.com/technet/security/bulletin/MS02-063.aspIt's an unchecked buffer denial of service exploit. To attack a client, they'd need to do it during an active PPTP session, and it is very difficult to make it do anything other than crash (i.e. run code). Probably not a big concern, but you should patch it up...