Try Atlassian's SourceTree.[1] I have only used the Mac app. It's the GUI I settled on after trying three or four others, including Github (again, for Mac). They built the Windows client after everyone who uses Windows at work clamored for it. I'm still on XP though and it requires 7+.
Atlassian also has a good, friendly set of tutorials for using Git.[2] The order in which they introduce the concepts and the commands encourages a clear understanding of the whole system.
SourceTree has been great and Atlassian's support is also excellent. However, I still can't work out how to turn off Git's overly-optimistic automerge, which leaves << HEAD strewn through files at random.
I briefly tried SourceTree when the Windows build was first made available for download. However, it too did not support Git submodules--although from my understanding the Mac version does. I'll keep an eye on it though, since that seemed like something that would be added in time.
Atlassian also has a good, friendly set of tutorials for using Git.[2] The order in which they introduce the concepts and the commands encourages a clear understanding of the whole system.
[1]http://www.sourcetreeapp.com/ [2]http://www.atlassian.com/git/tutorial