Windows 7 : need to leave desktop up and running 24x7

It's possible your IT department has some energy saving software in operation to power down machines over night in order to save on electricity bills.

That's what we do, we can add machines to be exempt but we'd need a really good reason for doing so.

Control Panel > Hardware and Sound > Power Options. On the left side click on Change when the computer sleeps, then change the dropdown for Put the computer to sleep: to Never.

I have the same necessity at work and I ended up using Don't Sleep. Whenever I need to keep the PC awake, I open it and send it to tray. It's freeware and suitable for Windows-7 x64/x86.