Norton "Backdoor.Trojan" virus warning

Norton AntiVirus posts a popup as follows:
Object Name: C"\WINNT\System32\msd.dll
Virus Name: Backdoor.Trojan
Action Taken: Access to the file was denied

I ran a full Norton virus scan twice, one in the "Safe" mode & one in Normal
mode. Both found the system clean with no viruses, but I continue to get
the warning popup. I'm unable to close the warning popup window. Restarting the system doesn't help.

This problem follows on the heels of a "Coolweb" spyware problem that I thought I'd cleared by downloading/running "cwshredder". It finds/clears
a coolweb problem, but it returns in several hours. The coolweb spyware also hijacked the browser page to one titled "about.blank". I'd run "cwshredder" and installed "Spyware", which now keeps my browser on my desired (Yahoo) home page, but as stated, cwshredder finds the spyware
recurring if I run it after several hours, though the browser stays on Yahoo.