Obligatory "Dude, where's your backup?" question.
I do not mean to sound cheeky, but, changing anything that installs drivers (yes AVs do install drivers) has the potential to mess up one's system. An external drive is not expensive. Get a nice 500GB one and provided you do not keep all your eggs in one basket, your C: drive should be 80GB max. A drive like that would go for many many many backups since apart from the first one the rest should be incremental.
No, you do not have to buy a disk imaging util, there are free ones out there. Macrium Reflect Free is one such product with lots of praises. (tho, the free version does not do incremental backups). Image your C: drive before a potential dangerous "move" and there, you're good to go. Why in this day and age people keep going "commando" with their systems, boggles my mind. Want fast backup for your daily office docs and whatnot? Dropbox gives you 2GB storage for free. Files get synced whenever they are modified and I think they have file version feature too.