Non admin accounts cannot run certain levels of execution required by many malware applications.
Technically, even running as an 'admin' is not really an admin in windows vista+ unless UAC is turned off.
Regular 'power users' can change most settings, install software, and modify local policies, but machine-wide settings often require an admin.
The safest way to run is in a protected 'guest' account that resets after use. In Vista this could be achieved with Microsoft Steadystate, but windows 7 provides similar features built in. Or, alternatively you can use a virtual machine only for bitcoin.http://www.microsoft.com/downloads/en/details.aspx?FamilyID=ef232619-7600-4768-b111-f60ba13862ea