Do they have a custom dialer program that runs under windows? If so, there might be some problems. But a standard PPP dialup only has a few things that are negotiated between the NAS (equipment you dial into) and your PC. You can capture this process with a decent sniffer, even a free one like Ethereal. Here's what happens, off the top of my head:
LCP negotiation goes through some basic options that are pretty much standard on all dialers: Async Contol Character Map (usually 000a000, Protocol Field Compression (yes), Address Control Field Compression (yes), Callback Control Protocol (no thanks), MRU (usually 1500), Authentication protocol (PAP or CHAP).
Once the NAS and the client agree on the LCP settings, the session progresses to the authentication phase. Your username and password are transmitted to the NAS, which in turn transmits it to the ISP's authentication server(s) using the authentication protocol agreed upon during LCP negotiation. If the server says your user/pass is okay, it sends some information back to the NAS that control your dialup session settings, such as your session time limit, idle timeout, static IP address (usually dynamic), compression scheme, Access Control Lists, etc.
After auth is successful, the NAS and the client negotiate again in the IPCP (Internet Protocol Control Protocol) phase, where the following information is determined: Van Jacobsen Compression on/off, IP address, DNS servers, and possibly NBNS servers if you are doing a microsoft dialup. Once all of the settings are offered by the NAS and the client accepts them, the session continues to the next phase.
The CCP (Compression Control Protocol) phase consists of the NAS and the client agreeing upon what compression scheme to use. For example, Microsoft PPC or STAC. Once compression is either negotiated or rejected, the session is ready to pass PPP frames between the NAS and the client. Now you can surf, check email, whatever.