If you have a Dell reinstallation DVD that will have all the required drivers on it for your machine. Make sure the COA code is readable, as you will need to reactivate windows after the install. You need to save any data that you want to keep onto (say) an external hard drive (all will be wiped by a clean install). You will need to reinstall any programmes you have (Office etc), as they cannot be copied across, and there will be a load of updates to download. Put some free antivirus software on before accessing the internet (I use MSE).
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