This one is a little tough. Your friend with this one is the registry. There is a key that tracks track installed software, however it is on a per-machine basis. The isn't necessarily going to tell you who installed the software.
What you can do is look for the registered owner of the software. This would be something the user entered during the installation wizard process. You could get lucky and have that field completed. You would look for "RegCompany" and "RegOwner" entries in the registry. This could be under the above registry key or under the individual registry key for the software you are investigating.
You could also look for instances of the msiexec process being run in each users' Event Log. The logs may tell you what the software being installed is, or they may not. This wouldn't work if the installer didn't user msiexec.
Another valuable registry key is the User Assist key. It is user specific and could provide you with the information you are looking for. The key is ROT13 encrypted, but there are a ton of parsers for this key on the web.
Various MRU registry keys (Most Recently Used) are a good place to look for programs being executed, including setup programs.
additional MRUs are referenced here:http://windowsxp.mvps.org/RegistryMRU.htm
Mounted Devices is another good registry key to correlate to various Link files you may find on the machine. You can even reference that against prefetch files based on the times. For example, suppose that a USB drive was mounted, and find a link file pointing to the Setup.exe file on the USB device. You can look for a prefetch file for most likely msiexec and see if it was executed around the same time.
For Internet downloaded software, check the users Internet History and Link files. If they downloaded the file, chances are they installed it. You can reference the above artifacts to confirm this.
Check out this PDF from Access Data for additional registry artifacts that may help you:http://www.accessdata.com/media/en_US/print/papers/wp.Registry_Quick_Find_Chart.en_us.pdf
When all else fails, search the entire registry for a list of keywords. It helps if you have access to a tool like Access Data's Registry Viewer.
<edit> I forgot to mention that the HKCU hive is the ntuser.dat file under each profile. The HKLM\SOFTWARE hive is in the WINDOWS\SYSTEM32\CONFIG folder, it's the SOFTWARE file. </edit>
Hope this helps.