What is it?
Nvidia Media Center Tray - NvMcTray.dll

What does it do?
NvMcTray.dll - This is loaded as a startup item. This is the tray icon to easily change a number of settings. Games like everquest actually need this to be running.

This is a windows startup item.? If you're looking at your HJT log you will see a line that looks something like this:
O4 - HKLM..Run: [NvMediaCenter] RUNDLL32.EXE C:WINDOWSsystem32NvMcTray.dll,NvTaskbarInit

Virus Precautions:
You'll want to keep an eye on this google search for any known viruses. The normal location of this file is C:WINDOWSsystem32NvMcTray.dll