*SOLVED* (for me)
I just received my Arduino Uno yesterday and had been struggling to get the driver to install on my laptop Windows XP machine (worked fine on my desktop WinXP machine).
I was getting the exact same problem as john.olsen describes (a few posts prev.). I would point windows to the driver location, it would copy it over, and then complain "The system cannot find the file specified". Arduino Uno would be listed in device manager under ports but it would be disabled, and no COM# was listed as part of the port name.
The fix that worked for me... (found here: http://forums.techguy.org/1680041-post1.html
1. Start -> Run -> "regedit"
2. Nav to: HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion
3. With "CurrentVersion" selected, choose menu option "Edit -> New -> Key"
4. Name the key "RunOnce"
5. Reboot (may or may not be required... I didn't try without rebooting)
6. Connect Arduino and install the drivers according to the instructions.
After doing this the installation went as expected. Hope this is useful to others.