Apple has released
iMac Update for Mac OS X, intended specifically for iMacs running Mac OS X 10.1.5 build 5T91.
To find out what version of Mac OS X is installed on your iMac, select “About this Mac” from the Apple menu. Then click once in the area that says “Version 10.1.5.” Mac OS X will then list the build number. If that number is 5T91, this update is applicable for you.
Apple said that this 2.5MB update “includes enhancements to support the installation of third party applications and the Networking Update which improves network and Internet access after restarting your computer or when waking from sleep.”
More details are available from Apple’s Knowledge Base at the link above.