Why do you not even mention what version Windows 7 is on the laptop?
If it's Ultimate you can use the Windows 7 DVD as it is, activating with the key on the COA sticker on machine (sometimes under the battery on Dells).
If it's another Windows 7 version, put the DVD in drive, extract the ISO using ImgBurn, run this tool on ISO:
Unlock any version eicfg removal tool
Then burn all-versions ISO to DVD to install your version with Product Key on COA sticker.