The copy of windows 7 is legit, i’m a poor student and can’t afford a new copy, any suggestions would be much appricated

I have a copy of Windows 7 business and no serial. I can not run a wallpaper and windows update dosen’t always work. Its a touch less responsive, but works better than a fresh install of Vista.

I am getting a new laptop which will be eligible for a free windows 7 home premium upgrade.

I also have windows xp on my desktop, will I be able to use the same serial number on my laptop and desktop?

Also, I have read that windows 7 uses less memory, etc and is faster just like xp.

I have installed Windows 7 Home Premium in my computer and activated it.What happens if someone stole my key.Can the thief activate windows 7 home premium on a different computer using the key which i am using already without contacting Microsoft?Can i activate windows if i reinstall using the same key after this?
@kingkrunal
thanks for the advice but i dont need serial keys as i have the genuine one.I asked this question because i was afraid if someone stole my key.