At first, copy your important information from Drive C into another Drive.
Installing Windows 7 & 8 are similar;
Insert Windows Disc into the DVD-Drive & Restart your system;
Press Enter when you see the Dos screen & Wait for the Windows to prepare setup;
Choose your Language & Keyboard and proceed to the next step;
There are two kinds of installation: Upgrade & Custom (Advanced), select Custom; Also there are two other kinds of installation (Architecture): x86 & x64 >> Select x64 if you've a 64-Bit based Processor and more than 3 GB RAM, otherwise select x86!
Select and format your System drive; then click next while the System Drive is being highlighted.
You're all done, just wait for the windows to be installed.
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If you still have problems installing your Windows, feel free to tell me