I think it just used the system UI font, so probably Segoe UI, which was pretty new at the time and Microsoft loved to use it.
(Specifically on Windows Vista and later. On Windows XP, it would still have been Tahoma.)
If it used the default document font for the chat window (which is likely), then it would be Arial in Windows XP, and either Segoe UI or Calibri in Vista and later.
I think it just used the system UI font, so probably Segoe UI, which was pretty new at the time and Microsoft loved to use it.
(Specifically on Windows Vista and later. On Windows XP, it would still have been Tahoma.)
If it used the default document font for the chat window (which is likely), then it would be Arial in Windows XP, and either Segoe UI or Calibri in Vista and later.
Source: https://learn.microsoft.com/en-us/windows/win32/uxguide/vis-fonts