Cannot run any programs or install on Server 2003 with Exchange

On the system I cannot open a CMD prompt.  It blocks me from running almost any program.  I ran a C: scan from another PC with Symantec AV Corp 10.2 and it found a bunch of viruses but they were all the same, (a virus related to R_Admin).  I deleted all and rebooted.  I cannot open REGEDIT either.  It is a PDC and Exchange 2003.  I can open Windows Explorer, AD USers and Computers and Computer manager to name a few, but not much else. I was able to ren=move some apps last week as I removed a anti-spam app that was causing problems.  I am an IT consultant (MSCE, CCNA 10+ years).  I recently took over an account and I think the previous company left a time bomb.  Any thoughts?

I am using the domain\administrator account.  It was working normally about a week and   a half ago.  I now think it is virus related.  I was able to install Symantec AV CORP 10.2 locally and it automatically found hundreds of viruses.  I have run full cleans and rebooted several times, it is still giving me problems.  The only way to run an APP is to Right Click, select Run AS, and the uncheck "run this program with restricted access"  I aom only having this problem on my PDC, all other servers are fine.

The virus I encountered was svchsot.exe:  http://www.symantec.com/security_response/print_writeup.jsp?docid=2005-010614-1404-99

Althought is says it is low, I think it let in other viruses or spyware.  My main concern getting my new email spam blocker in, my secondary concern will be long term I will probably need to wipe out the PC and start anew unless I can fix this.  

When I try to run anything new i get this message:"the files does not have a program associated with it for performing this action.  Create an association in the folder options control panel"

This happens on trying to open apps like AV.

Any insight woulb be great..THanks

I was considering that, I do have a few disks like that, the problem is that it is an exhange server and Exchange...I have cleaned it pretty good, i just still have that issue with running apps.