It depends on the OS and whether or not roaming user profiles are enabled.
For example, on XP, with non-roaming profiles, the location is
<SYSTEMDRIVE>\Documents and Settings\<user>\Local Settings\Application Data\Microsoft\IsolatedStorage
On Vista with roaming profile storage,
<SYSTEMDRIVE>\Users\<user>\AppData\Roaming\Microsoft\IsolatedStorage
See an Introduction to Isolated Storage for more info.