I'm using Windows 7. Or does Windows 7 already have that ability built-in?
The Time service has existed in Windows since Windows 2000, and has synced over the internet since XP. You can change the server if you're not happy with it's performance as detailed here, but the default always serves me well.
I use Atomic Clock Sync for my home PCs, though I really should just use the built in service as it would be one less thing to load on start up
Though I've just tried using the built in one on Windows-XP and I get an error - "An error occurred while Windows was synchronizing with [name of time server]." - for any time server I try. I asked a question and got an answer which solved the problem.
I actually went through this two days ago. Noticed my XP main machine was drifting a tad time-wise. I had used Peyo's Atomic Clock Synchronizer before and noticed it was still out there and working with XP. So I downloaded it from http://www.peyo-home.sk/atomclock/index.htm. Just works, although it has two quirks. The help button does not show any About information and certainly doesn't give a link back to peyo's site. And the exit button actually exits, rather than closing the interface and minimizing back to the system tray, as some of the other syncing programs do. Otherwise, it just seems to work on a schedule you set.
And like others, I have had issues with the Time Service in the past, thus the desire to find a good third party solution.