PowerShell access denied to cert:\CurrentUser\My

PS cert:\currentuser\my> dir  
Get-ChildItem : Access is denied.
At line:1 char:3
+ dir <<<<

I cannot access any of my code signing certificates to sign a PowerShell script. I can open MMC, Certificates, and see that my code signing certificates are installed and valid on my machine (Windows XP SP 3).

The other weird thing is that I cannot access any of my mapped network drives from PowerShell, but I can see them fine in Windows Explorer and a DOS window.

I've uninstalled PowerShell, rebooted, and re-installed PowerShell 1, but still have the same problem.

EDIT: My day-to-day account on this machine is NOT an administrator, but I do have an administrator account I can use for tasks that require it. The code signing certificate is assigned to my day-to-day account, and I should not have to be an administrator to sign code. I'm not sure how to give this account rights to the certificate store.

EDIT 2: I ran FileMon and RegMon to see what I am being denied access to. cert:CurrentUser\My is the folder C:\Documents and Settings\USERNAME\Application Data\Microsoft\SystemCertificates\My\Certificates. It also denied me access to C:\Documents and Settings\USERNAME\Local Settings\Temp. I can open Explorer and get to the files in these folders with no problem. I temporarily gave Everyone full rights to these folders and still received the access denied message from PowerShell.

Google hasn't revealed much. What should I do?

Message from Windows PowerShell event log:

Provider Health: Attempting to perform the GetChildItems operation on  
the 'Certificate' provider failed for path '\currentuser\my'. Access is denied.

Details: 
ProviderName=Certificate
ExceptionClass=ProviderInvocationException
ErrorCategory=InvalidOperation
ErrorId=GetChildrenProviderException
ErrorMessage=Attempting to perform the GetChildItems operation on  
       the 'Certificate' provider failed for path '\currentuser\my'.  
       Access is denied.

Severity=Warning
SequenceNumber=146

HostName=ConsoleHost
HostVersion=1.0.0.0
     .....