Regarding the use of a 2nd OS installation only for the scenario of disaster recovery, I do not think that is a wise usage of your disk space.
I'd just recommend having a BartPE CDROM handy in order to boot up and diagnose / fix your system when you need to. A virus cannot possibly affect your BartPE CDROM because it will be #1 Offline and inaccessible to a virus and #2 will be Read-Only because it is located on a CDROM.
A virus scanner can be added to the BartPE CDROM but just like with a secondary OS, the utilities and data will become old over time if not updated. So the benefit of a secondary OS on the HD vs a BartPE on CDROM is nullified. The best scenario would be the ability to create an updated BartPE somewhere other than your potentially dead PC if the occasion calls for it.
LHammonds