corrupt system file / recurring crash

At least once a month, my desktop crashes completely. The error message
indicates that the system file is corrupt, so I have to either try to repair
the installation, or reinstall from CD. Two questions:

1- how can I find out what "version #" I am running (it asks this during the
repair process - the field accepts two digits, I believe)

2- How can I avoid recurrence of this problem?

I back up important files regularly, and/or save to the D: partition, which
remains unaffected by this issue - but it is a hassle to reinstall so
frequently, and then to recover missing emails, etc.

Thanks!