Not 100% sure on the specifics between XP Home and XP Pro but take a look at this page:
http://www.microsoft.com/technet/prodtechnol/w inxppro/maintain/filesharing.mspx
What I'm getting at is the 'Security' tab on the folder/file/computer, and whether or not you have the correct permissions.
Some will let you create a file ("write") but not change it again ("append"). Make sure that you have "Full Control" of your PC. Seems silly to suggest that an Admin account would not, but thats the only thing I can think of.
I say this because on my USB drive I have given other users the very same ability (write, not append or delete) so they can't mess up my files!