Well, that post could handle your scheduled tasks.. but there's a UDF on my site that makes service management simple. Not as comprehensive as the UDF that Chris wrote, but (in my opinion) easier to use for Quick-n-dirty scripts.

The UDF on my site is called WMISvcMgr(). Here's some Pseudo-Code to put in a loop where $Computer has the name of the current computer to query.
 Code:
; Get a list of services on the named computer
$Services = WMISvcMgr('List', , , $Computer)
; enumerate the list of services, looking for those that use the old admin account
For Each $Service in $Services
  $SvcInfo = WMISvcMgr('Query', $Service, , $Computer)
  If $SvcInfo[8] = 'OldUserName' ; if the service account is the old user
    $SvcInfo[8] = 'NewUserName' ; update the new userID and password
    $SvcInfo[9] = 'NewPassword'
    ; stop the service
    $ = WMISvcMgr('Stop', $Service, , $Computer)
    ; write the updated config info back to the service database
    $ = WMISvcMgr('Modify', $Service, $SvcInfo, $Computer)
    ; restart the service with the new credentials
    $ = WMISvcMgr('Start', $Service, , $Computer)
Next

Realize that this is untested, PSEUDOCODE! It's damn close to working Kix code, but you should add some error checking and such.

Glenn
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Actually I am a Rocket Scientist! \:D