|If this is a desktop and you replaced the 'battery' then you replaced the CMOS battery. After doing this, you need to go into the BIOS set up and reset defaults, go back in and set time, date, boot drive, boot order, and probably a few others, depending on the motherboard/BIOS.|
On a desktop, everything relies on the power supply, like kuwese hints at, do not cheap out when it comes to power supplies. While you do not need the most expensive, a little bit of careful shopping can get you a really decent power supply for only a reasonable amount more than the cheapest out there.
You have to be a little bit crazy to keep you from going insane.