Right, I have been emailing back and forth with a nice techie at Kaspersky, and we have found a workaround for the problem where Vista practically grinds to a halt while scanning.
This is for Kaspersky Internet Security V7 by the way, version 6 doesnt have the facility to do this.
If you are running V6, you can upgrade here. (Version 7.0.0.125 at the time of writing this.) Directions on how to upgrade here.
Open Kaspersky, and go to Scan>My Computer>Settings>Customise and change the following;
On the General tab make sure these are set;
"scan programs and documents"
"scan new and changed files only - By Extension".
Then go to Heuristics Analyser tab and untick Root kit scan and
Heuristics. (These are optional, try it with and without, works fine for me with these enabled).
Now try a full scan and see if things are better, and if not, go to the screen where you can select the drives to scan and just choose your c:\ drive and try that. Your RAM usage *should* be much less than it was.
It's a workaround until they fix it.
You can also shedule a scan for each drive right after the other one, and that will avoid the high RAM usage as well.