Some updates, including KB885835 require more disk space on drive C: then I
have. Using the the /n switch (no backup) solved the problem until now, but
KB885835 seems to need even more space on drive C:.

I get the following message:

There is not enough disk space on  C: to install KB885835. Setup requires
a minimum of 5 additional megabytes of free space or if you also want to
archive the files for uninstallation, Setup requires 15 additional megabytes
of free space. Free additional space on your hard disk and then try again.

The relevant lines from the log are:

8.542: Allocation size of drive D: is 4096 bytes, free space = 10663268352
bytes
8.542: Allocation size of drive C: is 2048 bytes, free space = 31266816
bytes
8.542: Drive C: free 29MB req: 34MB w/uninstall 0MB
8.552: LoadFileQueues: SetupGetSourceFileLocation for halacpi.dll failed:
0xe0000102
8.763: Error: Drive C: free 29MB req: 34MB w/uninstall 44MB
8.763: Drive C: Need additional 5MB to install, 15MB with uninstall
8.763: DoInstallation:AnalyzeDiskUsage failed
8.803: VerifySize: Unable to verify size: Source = NULL: d:\winnt\oem32.cat
9.143: There is not enough disk space on  C: to install KB885835. Setup
requires a minimum of 5 ....

This is all correct, since my C: drive only has 30MB (just holds the files
for booting).

Is there any hidden switch to tell the update to use space on drive D:
instead of drive C: ?

Many thanks, Felix