Has anyone had any experience of implementing two factor authentication on a 2003 / XP infrastructure?

It’s something I’ve just started to think about, primarily for staff that access our MIS and finance stuff.

Are there any pitfalls I should look out for? Is there a single *best fit* solution? Is there any kit I should seriously consider and conversely suppliers / kit to avoid

Lots of questions I know but I’ll be totally honest I know zero about this aspect of network security

Any advice or opinions, good or bad, from those who have implemented two-factor authentication would be fantastic

Any links to suppliers, resources etc would also be great