How to make Steam NOT change your CD-KEY
1. Click Start
2. Search "Run"
3. Type in "Regedit"
4. HKEY local machine
5. Software
6. wow6432node
7. Bohemia Interactive Studio
8. Right click "Arma 2 OA"
9. Permissions
10. Click Advanced
11. Select Administrators
12. Click Edit
13. Check the Deny box on "set value"
14. Apply and click "ok" all the way out
NOTE: Make sure you changed your CD-KEY before doing this ^^
2. Search "Run"
3. Type in "Regedit"
4. HKEY local machine
5. Software
6. wow6432node
7. Bohemia Interactive Studio
8. Right click "Arma 2 OA"
9. Permissions
10. Click Advanced
11. Select Administrators
12. Click Edit
13. Check the Deny box on "set value"
14. Apply and click "ok" all the way out
NOTE: Make sure you changed your CD-KEY before doing this ^^

xD