What you need to do is:

1. Memory is better 2GB or more although it is not required.

2. Prepare 40GB or more for system drive although you can extend it later (Computer->Manage->Storage->Disk Management).  

3. Back up your data and

4. Install a fresh Win 7.

5. Remember to get all updates right after the installation is finished and the computer is connected to internet. (In search box enter "Windows Update" ->Windows Update-Check for Updates->check updates->install)

6. Don't forget Microsoft Security Essentials. Download and install it to make your computer safe.

 

Win 7 is very easy to install. You are garanteed to be happy with it.