Yup, sounds like CMOS battery. Very cheap and easy to do, shouldn't cost more than a couple of quid and you can do it yourself - but as previous poster says, have a a google. I'm assuming that as you don't know what this warning means, you haven't made any extensive changes to your BIOS settings and thus won't lose anything particularly important from them reverting to default (which they will do when you take the old battery out); you may need to reset the date and time but that shouldn't be too hard.
Basically, it'll be a large flat cell battery located somewhere on your computer's motherboard. Once you have taken note of any BIOS settings you care about (that's what that googling's for), turn the desktop off completely, take the side off, locate the CMOS battery on the motherboard and pop it out - you may be able to just grab it or it may be held in with a little clasp, designs vary. Take it to a computer/electronics/parts/similar store and buy a new one just like it, take it home and put it in, put the PC's side back on and turn it on. Go into BIOS while it boots up and deal with putting any lost settings back the way they were. Problem solved.
Thanks Have taken side off computer - and hoovered it! Noted down battery number and make, and have bought one from ebay. perfect. Just need to change them over. Looks like it's held in with a clasp. Feel quite proud of myself!! Thanks