Did XP come preinstalled on the PC?
If so it qualifies you for Upgrade version Windows 7.
If you installed Home Premium you can buy an Upgrade key for it which we will help you activate following Clean Install with a Upgrade Windows 7 Version.
Very few consumers need the more expensive higher versions which offer just a few extra expensive features but otherwise are the same OS.
Technically there is no trial period and you should not install Windows 7 if you haven't bought it. But considering you had a bootleg copy and are trying to make it right this is probably your best course.
Look over these tools and methods which work best for getting and keeping a perfect Windows 7 install and are the same for retail: Clean Reinstall - Factory OEM Windows 7
To help conserve resources I'd also follow these steps to Optimize Windows 7
especially editing Visual Effects of animation, fading, sliding, dragging intact, and selection rectangle which are huge RAM hogs.