Step One:

Check Event Log:
Check your Event Viewer for these errors...
MSDTC errors (4427,4691,4135 are common error events for this)
W3SVC Event 36 "The server failed to load application '/LM/W3SVC/1/ROOT/'. The error was 'Class not registered"




Step Two:
Open "Control Panel" and then "Administrative Tools" then Open "Services"...

Locate the "Distributed Transaction Coordinator" make sure the process is "Started" and "Automatic"
(If its not, open a command line and type "msdtc -install", then try and start the service)

Open a command line and type the following...

cd %windir%\system32\inetsrv
rundll32 wamreg.dll, CreateIISPackage

(NOTE: "CreateIISPackage" must be typed exactly; it is case-sensitive.)

regsvr32 asptxn.dll

(wait for a dialog box to notify you asptxn has registered correctly)

Then, in the cmd prompt window type IISRESET

Open "Control Panel" then "Component Services"...

You should see all three IIS COM+ applications that have been recreated


Finally, open "Control Panel" and then "Administrative Tools" then Open "Services" and just make sure that

DTC
COM+
World Wide Web Publishing

are all running.....

As a very final check just stop and restart the web service in IIS itself.

With a bit of luck the COM+ and ASP/ASP.NET issue should be resolved.