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#11
If this is the HP in your specs then use the Product Key on COA sticker to Clean Reinstall - Factory OEM Windows 7 .
If it's retail Win7 you've installed then the Product key should be on the disk packaging or in an email if you purchased a download, which can also sometimes be tracked by customer service from seller. Otherwise you'd have to audit it from the running OS using Belarc Advisor.
To start Win7 I'd burn the DVD or write to flash stick, boot to try the repairs in tutorial, rescue files if necessary.