I recently encountered a strange problem accessing my SharePoint0 sites and the SharePoint Central Administration site after installing .NET Framework 3.5 SP1 on my Windows Server 2003 running SP2.
The error message I recieved was the HTTP 403 Forbidden error. This was showing up for all sharepoint sites.
At first I tried specifying the URL along with the .aspx page name, resetting IIS, checking IIS to see if Integrated Authentication was enabled in IIS, and a bunch of other troubleshooting steps, with no luck.
I then decided to uninstall the .NET Framework 3.5 SP1 using the Add/Remove Programs, but strangely the problem persisted.
Solution:
Finally after some googling around, this is the workaround I used to fix this problem. I'm not sure if this is the best way of going about solving the problem, but it worked for me.
1. Open IIS (click Start -> Run -> and type inetmgr)
2. Expand Application Pools
3. Right-click Application Pool of your web application and click Properties.
4. Click the Identity tab in the property page
5. Under Application Pool Identity, choose Local System instead of Network Service
6. Click Apply
7. Click OK
8. Reset IIS