Question: I've installed TFS 2008, but I can't seem to access the server. When I try to connect to it in Visual Studio, I can't. If I try by browser on a remote PC, I get a generic page cannot be displayed. On the server, I get a 403. Nothing was touched in IIS and the service is running as a Network Service. Any ideas?

Answer: 
Turns out the time and date on my computer was not "close enough" to the time and date on the tfs server. Changed my system clock setting and problem went away.
I went through everything on a similar problem.

I logged onto my tfs server and connected directly there.

I also used a TFS admin tool I downloaded some time ago from Microsoft, and made sure I was in all the right groups and projects.

I then went back to the client PC with the problem, tried the services/1.0/serverstatus.asmx?op=CheckAuthentication Url again, and it worked this time.

AFter that full service was restored to my PC.

So I don't have the exact answer, but I would go through the checklists presented by Grant Holliday in his answer.