Gateway 7405gx boot failure

I have a Gateway 7405GX that started having boot problems over a year ago.
At first it was a very sporadic problem but over time happened more and more.
As I recall, it would not boot and give an error regarding the boot device; I could hit F1, enter BIOS setup and then exit and the system would boot fine. Running a chkdsk showed no errors.
So, for a year or so I would use the system this way, until the problem advanced to a point where I can’t get into BIOS or boot.
I figured the BIOS or mobo was the problem and replaced the system. Lately I’ve started wondering if it could be as simple as replacing the CMOS battery, something that seems relatively inexpensive.
I feel like I never got my money’s worth out of this machine and it’s the only machine I’ve got that’s compatible with my Protools setup, but want to fix it myself and don’t want to spend hundreds on it.
Any ideas MUSC appreciated.
Thanks!
Dan