Hi.
My {systemdrive}/Windows/Installer dir is currently 4.9GB big - making my Windows dir sum up to ca. 21GB.
I'm running Vista Business SP2 32bit.
A similar (but old) thread is here.
Ran msizap g from command line as it was suggested here but no changes.
The folder itself contains lots of .msi and .msp files and some folders, but the later take only ca. 50MB.