How can I get around "error 0x80070522" when creating files in the root of the C drive (C:\)?

One of our customers has just found a problem when trying to create a file on the root of the C Drive (C:\), on a Windows 7 Professional PC.

I know they shouldn't be keeping files here, but there is a valid reason in this case, so I've relaxed the security on C:\ by giving the users group users modify permissions.

Before I relaxed the security, the user was receiving access denied, but now they are receiving the message:

An unexpected error is keeping you from creating the file. If you continue to recieve this error, you can use the error code to search for help with this problem. Error 0x80070522: A required priviledge is not held by the client.

Googling for this suggests that it is caused by UAC, but how can I get around this when the user doesn't have admin rights on their PC?

It's a security setting in UAC. Basically, even if a user has permission to write files to the drive, UAC still prevents it since UAC has a rule to prevent non-admins to writing to the root of C:

Only reasonable workaround is to disable UAC, which then creates a security risk.

Best bet is to have them save to another location.