My development machine have MySQL server 5.0 installed and i upgrade to version 5.1.
After i've installed MySQL server 5.1, i can't found the data folder which normally in "C:\program files\MySQL\MySQL Server 5.0\data" for version 5.0.
Finally able to get the location in in "C:\Users\All Users\MySQL\MySQL Server 5.1\data" for Windows Vista
If you are using windows XP, it's in "C:\Documents and Settings\All Users\MySQL\MySQL Server 5.1\data"
Hope this help