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Originally Posted by Incognitas
Had a look and it's enabled.
I also restarted the PC and it seems to be at the correct time.
Will let you know if it happens again.. 
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That's cause when you boot, your 'puter is using the time/date using the CMOS clock of your PC, but when Windows is up and running, it uses it's own mechanism to try and calculate the time. The CMOS clock is usually correct (unless the battery is on its way out), whereas the Windows one is often wrong.
Computers today use a device equivalent to an Intel 8254 timer-counter chip. The timer-counter is programmed by the BIOS to generate an interrupt every 54.936 milliseconds, or about 18.206 times per second. Another BIOS routine counts the interrupt requests and generates a time-of-day clock that can be read or set by other software programs.
Windows XP is a lot more consistent than 9x ever was (thanks to the w32time service) but it's still not 100% accurate.