Moving from VSS to Subversion is going to be already a great upgrade. Subversion will give you great features like atomic commits, great GUI, integration with IDEs, great Windows experience, the concept of a changeset, reliable repository, etc. For a typical Windows based company with small/medium teams, I think subversion is a great tool.
If you are interested in a Distributed VCS, then you should be looking at git, hg, bzr. hg and bzr are ahead of git as far as Windows supports. However, there is a ported command line version of git for Windows (msysgit) which merges back changes to main git. Also the git community is growing fast and hence I expect that the Windows experience will improve.
Git supports a hybrid scenario where the server can be CVS / SVN and individual developers can use git-svn to work locally and manage local branches. This kind of setup gives the best of both worlds. However, git-svn is flaky on Windows due to a dependency on Perl SVN libraries. In this scenario, it is not as easy to use nice features of git like developers sharing branches, etc.
Given that your projects are not open source, I think Subversion is likely to provide all the features you need. Once Git is up to the Windows usability bar that you need, then you can import your SVN repos into Git.
Unless your company is heavy on branching and merging, I would go for SVN, otherwise, you will need to consider a DVCS closely.
I have been working on an open source project, the repository was stored in CVS, which then got migrated to SVN, and then to Git. Every step was a major upgrade. The major motivation for SVN to Git move was to make it easier for contributors to maintain their own branches, keep them up to date, push them to developers, and for developers to easily apply them with minimum effort.
