Although it’s usually not my preferred OS, I had to implement a file synchronisation (synchronization) on a Windows XP (R) system.
The user wanted a real sync between two folders (Hard Disk and USB Drive).
Means: When a file is deleted/created/renamed on the harddisk, it should be deleted/created/renamed on the USB drive, too - and the other way round..

What I found so far:
Allway Sync (nice gui, easy to use, limited to 20.000 files per month [freeware edition])
SyncToy (from Microsoft (R), not tested yet)

Any better ideas?