DNS issues over VPN

I have many users connecting over VPN (PPTP using Microsoft VPN) and all of a sudden only some are not able to connect to DNS. They can ping using IP addresses, but not computer names. When they use the nslookup tool it gives their ISP’s DNS server, not ours. They seem to connect to the VPN just fine, though. I looked in event viewer and there are a couple of warnings that say the NTP client couldn’t sync the time up with the domain controller, but the last one was this morning and users are still having the same problem. No configuration changes were made at the time when people started having this issue. We are working with routing and remote access on a 2003 server. Any help is much appreciated!