Having done this upgrade on 3 different PCs/laptops now, I can confirm that the upgrade from Vista to 7 is very simple - as others say, just ensure you've backed any important documents up - usb/cd or external hard drive, your choice.
XP to Vista took a little longer as there were one or two compatability issues that had to be ironed out but the PC is running like a dream now on Windows 7.
In short - I'd thoroughly recommend doing it - far, far better OS than Vista!