you should have created recovery cds when you got the laptop.
Make a backup of your system [save important files, images etc to another hard disk,usb,computer] in-case you mess up things.
1.Now you have to download whatever win7 version you have installed.
Not pirated iso
Source i mean:
Windows 7 Home Premium 64-bit Download iso
1b. burn iso to dvd.
2.Install via dvd.
3.It will ask for activation after first start or After installation right click my computer>properties>activate.
you must be connected to internet to activate.
I tried this on friends laptop and it registered , activated successfully but you must download the win7 version that came with laptop[Important].