Windows.com time is very accurate. If you want the extreme of accuracy, there is an alternative in the dropdown box where you do the time check on XP and that is NIST. com time NIST is National Institute of Science and Technology and they link to worldwide standard time setting.
If you machine is unable to keep accurate tim it is the machine at fault, not the references.
Note that the use of timing in a computer is to give a refernce base on which the circuits operate. Telling you that it's half past nine (or whatever) is incidental to its real function and there is no guarantee (or need) that your computer is at all accurate, it just need to be constant.