How do I reformat my computer with Debian but keep the .debs I recently obtained through apt-get?

I tried installing a couple things from source recently, and it appears doing so crashed a few apps for good. I can't revive them, so I figure I broke parts of my system.

Now, I have the drive partitioned like this:

/hda1/ = /
/hda5/ = /home
/hda6/ = /usr
/hda7/ = /var
/hda8/ = (free space: 20 GB)

I want to reformat 1, 5, and 6, but I want to keep the packages I've aptgetted in /var. In the past I would have weird issues such as some error about HAL and whatnot if I manually tried editting fstab to get a reformatted OS to accept what's in hda7. So, how do I get the reformatted OS inside the drive but keep my old var?

If it gives you errors, I can't really see a way around it. Perhaps you could copy them to a removable disk and then copy them back again, or if that is not fiesable, create a directory under /home, copy them there temporarily and then move them back across, or perhaps utilise that empty 20GB partition there.

Using Arch, I personally create a seperate partition for the cache directory (/var/cache/pacman/pkg). Then I mount this partition under the correct folder. This saves loads of time when reinstalling, which I do loads, because I often get bored and trash my system (last reinstall was this morning ). Whenever I format, I just avoid formatting this partition. Works every time.

Have you tried just making a back-up of your debs and then moving them back after formating? Alternatively, you could instruct the install to install /var on your / partition, then edit it to use the partition you already had. That should work, I have done similar things numerous times before, like moving an install to a different part of the drive or even an entirely different drive, or putting swap, tmp and var on a single partition. It seems to me that much depends on how it is done. I did run into permission issues the first times I tried it because of using the wrong options to cp.