PC Problems

[punkerdude]

Around 2 months ago, whenever I turned on my computer a message right after the bios screen appeared indicating my CMOS battery was running low and requiring me to type the time and date on the bios everytime I turned the computer on. I had no big problem with this, but around 1 week ago when I installed the retail drivers from ATI my pc started to freeze randomly (i used the default windows xp drivers but i needed to use some extra features that i could get to work with those), either at the bios screen or at the loading windows screen or whenever (i couldnt get past 10 mins using it without freezing) and after this happens i must turn off the pc but when i turn it back on it shuts down like 3 seconds after (nothing gets to be displayed on the monitor).
I replaced the ATI card (AGP) with a nvidia (pci) both are 128mb but the same seems to be happening. It is very annoying as i cant use that computer anymore, and i dont know what more to do can anyone give me some ideas on how to fix this or what could be the problem?

In summary:
1.Computer works fine
2. 2 Months ago I get low cmos battery message
3. 1 Week ago installed ATI catalyst drivers (dont know if this caused the problem)
4. 3 Days ago computer starts freezing randomly
5. Replaced the ATI card with a Nvidia
6. No results