Well I can tell you broadly how to check (AFTER you've installed Windows 7).
1) Go to the Control Panel, click on System, scroll down the system properties page. Right at the bottom it should show the activation status- If that entire bottom part has vanished i.e. there is a blank space there, its a tell tale sign of a particular crack.
2) If after installing windows, it is automatically activated without entering a key, is likely using a crack. You can uninstall some cracks this way- boot to the windows DVD and select Repair PC. Navigate to the command prompt and type these one by one (hitting Enter after each).
bootsect.exe /nt60 all
3) Validate windows online at MS's genuine website or run this tool mgadiag.
Run the tool, click on Copy button, paste into notepad, save as .txt file, upload file here.
Bottomline is as long as the disk is free of malware, other things can be handled. Just uninstall the illegal key with a slmgr command, run a sfc scan, then insert your genuine key and activate and validate.