From my own experience....DONT DO IT! VLK is correct.
If you need to reinstall Avast, you must be running the OS to which it will be "partnered"
In a dual boot situation, this does not occur because of registry changes.
The problem that went unresolved with my system was WHY my computer crashed after Avast was installed. A complete reformat of the system, and restore of W98 was needed. Avast and W98 have been happily living together again.
I would assume that my W98 was on the way out and some conflict arose with the drivers in Avast that pushed it over the edge. Not really the fault of Avast.
Avast likes things simple. Try not to confuse it with dual boots and sharing registry entries.
Good luck.