But as I dont have an o/s to install...will formating just wipe the memory or the o/s as well???
A format does what it says on the tin (and as I hinted above), it wipes an entire drive.
As I said above, please don't waste your time trying to clean up Windows. You'll waste much of your life and probably get nowhere in the end. Believe me, I've done it once or twice as a task in the historic past for people when I was being paid by the hour to do it. Its a thankless, frustrating task and you'll never really properly clean out the nooks and crannies of Windows. There is a reason why a wipe and re-install is an oft recommended solution to many ailments.
If you value your time at all and are insistent on re-using the laptop, just go buy a copy of Windows 8 (or if you don't want to spend money, perhaps try contacting the laptop manufacturer and see if they can send you a Vista disk). Vista was a pretty useless operating system anyway.