Not enough disk space when installing Windows XP SP3 on Mac boot camp

I got this message when i tried to install SP3 on a Windows XP SP2 running on a Mac (boot camp): “There is not enough disk space on C:\WINDOWS\$NtServicePackUninstall$ to install Service Pack 3”

Solved the problem by adding a new string value named BootDir with the value C:\ under HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Setup\. Restarted and installed SP3.