By default Windows Vista will only synchronize with a time server once every seven days. If your PC’s clock is not so good at keeping the time or you just want to increase the accuracy of your Windows time you can decrease this update interval by editing a registry setting:

• Open up the Start Menu in Windows Vista
• In the ‘Start Search’ box type Regedit and press Enter
• If you received the Windows UAC ‘Windows needs your permission to continue’ prompt click Continue
• In regedit navigate to HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\w32time\
  TimeProviders\NtpClient
• In the right pane, right click on SpecialPollInterval and click Modify
• Select Decimal instead of Hexadecimal.
• Type in how often you want it to update in seconds and click OK.  - Do not change the interval to any value less than 14400 seconds (4 hours) or your computer’s IP address might be blocked by the time server.
• Close regedit