Yet another mystery solved..
I got the very annoying error message that my disk space in C:\Windows\$NtServicePackUninstall$ is full.
But there was more than enough disk space on my fresh installation of Windows XP on from a bootable USB stick.

There is a workaround that was originally only intended for iMac Users who switch to the dark side
But it works pretty well for Lenovo Ideapads (and mybe some other Netbooks that run into this message).

Before you install Windows Service Pack 3 follow these steps then:

Step 1:
Press “Win”-Key + “R” (alternatively Start -> run) and enter “regedit” and press enter. This will bring up your registry editor. Be aware that this is the moment where you can do serious damage to your windows software installation. So don’t mess around if you don’t know what you do.

Step 2:
Navigate to
HKEY_LOCAL_MACHINE\Software\Microsoft\Windows\CurrentVersion\Setup

Step 3:
In the right pane, click right mouse button and “New String” or (“Neue Zeichenkette” for Germans).
Name it “BootDir” and give it the value “C:\”

Step 4:
Close regedit and retry installing Service pack 3.
It should work now.

Please drop me a comment if I could help you.