rklm, sorry, seems a hardware or, at least, BIOS issue...
I did a complete scan with Avira recovery CD but to no avail. Booting Knoppix works and I can access files on the disk which used to be C: and the partition table seems ok. I tried various BIOS settings (switching off UDMA, reset PnP settings etc.). I even unplugged other IDE devices but no effect either. A demo version of a disk test tool at
http://www.german-sales.com/hardwaretest.htm wasn't really helpful, although a strange thing happened: the tool claimed that the read test on hd0 was canceled by the user although I did not do it. This did not happen with the other disk (both are larger than the BIOS regularly recognizes - 160GB and 250GB).
Is there some way to find out whether there is a BIOS error? And if there was a BIOS error, in which way could that be related to the avast update? This is really weird...
Appreciate any ideas what else to do (other than carrying the PC into the next service shop).
Cheers
robert