We've worked with several 3rd party products that allow users to log into their desktop machines whilst away from the office.

• Log me in
• Real VNC
  (Virtual Network Computing)

These products basically fulfill the same role as Remote Desktop Protocol which is built into Windows XP Professional. We opted for this as:

• It's supported by Microsoft, whom we rely on for other application support
• It's built into XP, therefore has a zero marginal cost (effectively free)