I suggest that you set the internet time source to one of the ntp.org pools - e.g. uk.pool.ntp.org in the UK. Time.windows.com is... errm... not the most reliable time source, putting it kindly.
When Windows boots it first obtains time from the BIOS, so check that is set reasonably accurately, because (depending on configuration), if the offset between system time and ntp source is beyond the allowed value, Windows will ignore the ntp time as being inaccurate. Of course, you can configure this to accept the ntp time whatever the difference, but it's not the default.
Here's a good MS article on the time service:
How to configure the Windows Time service against a large time offset
As an aside, if the time setting on a Vista or above PC is wrong, and not set to update from an NTP source, it will prevent Windows Update from functioning.
SD