By way of an update to this thread, Igor and Vlk have been helping me with this problem through 'My Messages'.
They enabled me to crash the PC and create a full dump, which once analysed, pointed to a conflict being caused by
C:\WINDOWS\System32\Drivers\Klif
which is a Kaspersky driver.
I've
never installed Kaspersky (just run the online scan which apparently uses ActiveX, not drivers). Further investigation by Igor and Vlk revealed that this entry, and others relating to Klif come from ZoneAlarm, which it would seem, use the Kaspersky drivers.
I've started a new thread
http://forum.avast.com/index.php?topic=31607.0 asking if anyone using ZA could possibly check their system to see if they have similar entries.
Why these drivers caused Avast (as well as AdAware and Counterspy in the end) to freeze during scans if I had the 2nd HDD installed, but caused no problem if I disabled it first, I do not know!
I would however like to take this opportunity to thank everyone on here who has helped with this, but in particular Igor and Vlk, who have put in an incredible amount of work in order to rectify this problem. I'm still a bit lost for words every time I think about it. I've been helped a lot on these Forums before, but this time it's just been amazing. I really can't believe the level of commitment.....I'm serious, what do I say?
Thank you.