I cannot step into .NET Framework source

Question

It's getting really frustrating. I can't step into .NET framework source (and at this point I am interested only in looking at System.Web) no matter what I am trying.
So at work I have VS2008 SP1 + .NET Framework 3.5 SP1 on a XP SP3 machine and on my home machine I have VS2008 SP1 + .NET 3.5 SP1 and VS2010 + .NET 4 on Win7.
I have tried every solution out there, setting the symbol server to http://referencesource.microsoft.com/symbols checking/unchecking the proper options in Tools-Options-Debugging-General with no use. The pdb files get downloaded and cached, and they are loaded when debugging, the module window says so, but when I want to go into the source code I always get "there is no source code available for the current location"
I even tried to set the symbol server locally (http://referencesource.microsoft.com/downloadsetup.aspx) with no luck.
What's really annoying is I remember at some point I was able to step into .NET code.
Can someone give me some hints as why this happens, where to look? I know there are tons of posts related to this particular issue but most of them are kind of old, 2009 and earlier, and I don't know if those solutions still apply or not, although lead by despair I tried all of them.

Thanks in advance
iulian