Loosing time when the computer is switched off is the first sign that the CMOS/RTC battery is going flat. If you leave it much longer, you run the risk of also loosing your CMOS settings, so get it replaced as soon as possible. Look at the battery on your motherboard to get the exact model number - It will probably be a "CR2032" that you will need to ask for. [BTW: This is code for "Coin cell", "Round", 20mm diameter and 3.2mm thick.] Other advise give above is correct.
acidrain, if your minutes are always correct, then it doesn't sound like a CMOS battery problem. Have you checked that your time zone is set correctly? By default, Windows XP and later will sync your computer time with an internet time-server every now and again, so if your time zone is wrong, then the hours will be set for that time zone.