I assume you installed the appropriate Windows 7 Scanner software
HP says ...
1. Click on windows icon or start.
2. Click on search and type ‘scan’.
3. Find ‘scan a document or pictures’ in the above pane.
4. Click on ‘scan a document or pictures’.
5. New window will open try to scan from that windows.
Their site also has These
There's even a Better/Easier way if you have Irfanview