What is it?
Nvidia Control Panel - NvCpl.dll

What does it do?
NvCpl.dll - This is loaded as a startup item. This loads the Nvidia Control Panel which is where you can tweak a ton of settings and overclock your card. Without this your drivers will be rather messed up and may or may not work properly.

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: [NvCplDaemon] RUNDLL32.EXE C:WINDOWSsystem32NvCpl.dll,NvStartup

Virus Precautions:
You'll want to keep an eye on this google search for any known viruses. The normal location of this file is :WINDOWSsystem32NvCpl.dll