This is not postgres problem but i cannot find a place to assign "total control" or "write" access to user "postgres" to a specific folder. I don't remember if i can do it on windows xp sp1, but this means that all people that have win xp sp2 can create databases on default table spaces only; or can create tablespaces only on a shared folder (common place to all users who can write, delete, etc).
Later if i find something about this it i will post a message.
If you're on XP Pro, then you should be able to modify the security on the folder by using the Security tab on the properties page for the folder. Failing that, try using cacls.exe which should be on any XP machine.
Regards, Dave.