Which OS are you trying to install? From what you are describing I am guessing at Windows 7 but can you confirm this.
Also are your HDD's and DVD being recognised in BIOS?
Which SATA channels are you using?
Lots of questions but no answers yet..... sorry! 
Yes, I am trying to install Windows 7, preferably from USB, but right now I won't be able to even (re-)install Windows XP

You ask good questions!
So here goes:
There are six SATA channels controlled by the ICH9R chip.
The two (SATA types) HDD's are on SATA channels 0 and 1.
These are configured as AHCI.
Both HDD's are recognized in BIOS.
There are two SATA channels controlled by the GIGABYTE SATA2 chip.
The DVD (also a SATA type) is connected to one of the GIGABYTE SATA channels.
These are configured as IDE.
The DVD is also recognized in BIOS.
Legacy USB Storage Detect is enabled. I suppose my board supports USB booting?
Thank you for taking the time!
