its not definitively the cause but most likely, changing the battery will only cost a pound or so and wont do any harm, just make sure you have all your BIOS settings written down on paper. then if it still loses time then its time to think again. H.
Um, an amatuer guess here but might this be something to do with sync'ing the clock with an internet time server? It may not be set up correctly.
My help centre says a few para's, here's one:
If synchronization is enabled, your computer clock is synchronized with an Internet time server once a week. However, if you don't have a continuous Internet connection through a cable modem or DSL modem, the automatic synchronization might not always occur. In that case, you can force an immediate synchronization by clicking the Update Now button on the Internet Time tab in Date and Time in Control Panel. This tab is only available if your computer is not a member of a domain.