Regarding Win2000, on 4/26/00 I asked the same question of Diane Mailloux (Product Manager for SilkTest) and her response was:
SilkTest 502 has official support for
Win2000 Release Candidate#2. This was the latest version when the release
came out at the end of the year. Win2000 (Commercial Release) will be
officially supported in SilkTest 5.0.3 due out end of May.