Where did you get OEM disk from? The same OEM? If not, your COA keys (printed on the back) probably will not work (I don't know, will not work on XP).
Check whether your copy is activated (Control Panel -> System). If not activated, then you can recover easily by using internal Windows commands.
1. Download win7 OEM certificate file and save in C:\. It must be matching SLIC OEM string in your BIOS.
https://docs.google.com/open?id=0Bxj5NEo7I3z9dWx3VndfenZBWVE
2. Download Win 7 Pro SLP OEM key. Any Royalty OEM Windows 7 Pro key will work. You can use even Microsoft key (published on Microsoft Web site). Note, you will be not using COA key printed on the sticker.
https://docs.google.com/open?id=0Bxj5NEo7I3z9dWx3VndfenZBWVE
3. Insert certificate and key. Run from command line as administrator. Wait for UAC popup between:
slmgr.vbs -ilc C:\your_filename.xrm-ms
slmgr.vbs -ipk xxxxx-xxxxx-xxxxx-xxxx-xxxxx
(replace xxx's with your OEM key
4. Verify activation:
slmgr.vbs -dli
EDIT: In the first step you might have to remove existing OEM certificate from the system. Not running Win7 at the moment, check command options by typing
slmgr.vbs -?