Well, first let me tell you that depending on the type of Windows Setup disc, there is still a possibility to have security tools installed before Avast. I'm not saying this is your case (since I don't have any way to know it), but the possibility is there.
So this means that there might be some other security tool, whether "pre-installed", or that was installed "anyway" (without your knowledge, and even if it is not listed in your "Programs").
Now, besides this, are you able to say the problem is Avast itself? Wouldn't this be happening in your system, with any other antivirus, too?
Do you have any free space in your first disk? The reason I ask is so you could try running the specific removal utility for Avast (
http://www.avast.com/uninstall-utility) under Windows Safe Mode, and then install Avast anew in the first disk.
I know this may sound at least "strange" to you, but separating the programs from your OS in 2 different partitions is not giving you any real practical advantage under MS-Windows. Using other OSs, it may. In theory, it is possible to have 2 Windows installations with dual boot, and to have only one (and the same) installation of Programs. In practice, it is not stable enough (to say the least).
So, if you could try the removal utility for Avast under Windows Safe Mode, using the correct version(s) of Avast (run it more than once if you ever had other versions of Avast), and
selecting the correct current installation folder of your current Avast in the removal utility, maybe you could try installing Avast in your first disk?
After installing, please reboot immediately (before configuring or updating) and only then proceed to test, configure, definitions update, ...
Independently of that suggestion, I would double check for any "strange" driver ("Device Drivers -> "Show hidden devices") and services, so to find out any potential security tool that might have left there some remnant (coming from your installation discs, even when you might have uncheck any pre-install software).
Whatever the case, please report back.