Ok, heres the problem:
Ive got an old pentium pc (old i know) but the date on it is completly wrong. I know its not the milleium bug since it can go past the year 2000, but even if I set it to thw right date when I switch the pc off it doesnt like contiune the date 
.
Im thinking it something to do with the battery thats on the motherboard, does anyone know where you can get replacements? If the date and time keep resetting then you are correct - it is probably the battery on your motherboard that needs to be replaced. These batteries aren't necessarily difficult to find, but you will need to find out the exact type of battery your motherboard uses before you buy one - either see your motherboard's manual, or remove the battery and physically check it. It is likely to be a "CMOS Battery" but I believe these come in different voltages.
Try this site for a replacement.
Hope this helps! The CMOS battery is most probably already worn-out. You should buy a new one. As far as I know, it does not cost too much.
The best thing to do is to search for a decent computer hardware store where you could basically get everything that you need. The term is "CMOS batteries" as previously mentioned. If you are unsure of what you are buying, simple take out the CMOS battery from your motherboard and then go to that particular computer shop. Show the CMOS battery that you have and have them look it up for you. Installing it is very easy too. Just pop it in and you'll have a working, stable system again.
Cmos batteries are 3.5v.
They are soooooo simple to replace.
Open your case and look for either a silver disk, this is a large watch battery that's all. Or if it's really old it could look like the size of a long match box with a black and red wire coming from it and plugging in to your motherboard via a 2 prong connector.
Changing it is very very easy. Don't take it to a repair shop unless you know they're not going to charge you a huge repair bill as it only takes 2 minutes to replace.
I hope this helps.
Usually, unless you've had the machine turned off for a year or two, the CMOS Battery should remain charged.
Keep them away from children, mind 
*gulp* i'd agree; sounds like a classic case of a dead bios battery. i've never replaced one tho; hope it doesn't cost too much.
I managed to pick one up for about $10 (if my currency conversion is correct!)
Jst replace the CMOS battry
Yep, that's one of the easier to fix computer problems. I recommend getting the number off of the battery, and searching it in www.froogle.com sounds like a dead battery and if thats not it, boot into the bios and change the clock and date there, see if that helps.
good luck
It's also always a good idea to make sure you have the latest version of the motherboard BIOS installed. Go to the motherboard manufacturer's Website and you should be able to find both the latest version and the software you need to install it. The version of your current BIOS is usually shown immediately after switching on your PC.
show it to a guy @ the local hardware store and he will probably have 1! lol
