It seems counterintuitive, given that Apple and Microsoft have a longstanding and fierce rivalry, but yes, you can control a Windows 7 computer via an iPad. As a matter of fact, there are multiple options for controlling a Windows PC via an iPad.
Microsoft Remote Desktop is an app available in iTunes that serves this specific purpose. It's made by Microsoft and is free.
Parallels Access is a very favorably reviewed app that does the same thing, but costs around $80 US per computer, per year. However, in addition to Microsoft's offering, another free option called TeamViewer is rated well by CNet and is free for personal use (but not corporate uses). It not only works on iPads but also iPhones, Android devices and some Windows phones.