Q: what is a rootkit?

guys,could you please tell me what a rootkit is.how does it affect my system,and is it safe to erase it whenever avast detects it?its bugging me everytime i use my pc.along with this worm/virus located in system32/x.whenever avast detects it i also erase it.i got pissed off one time while im in the middle of a game and it suddenly pops up closing the game cause i cant use alt+tab while playing SF.it closes automatically by doing so. how can i get rid of these persistent pest?i still cant afford to buy registered versions of softwares, i just download free stuff on the net whenever i think they could help me.antivirus and download accelerators.

i just recently had my newly formatted pc and my friend cloned a harddisk for me.it didnt have any antivirus and when i connected my internet these guys are back again.how do i battle these hard-to-get-rid-off minions?

please suggest stuffs that could best protect my unit.i got lots of things to learn about my system.if i could only afford to buy all those softwares and services ill do so, but i am still not able for now.help me guys T_T

first, avast isn't as good as it used to be. It's fallen behind on benchmarks in the past year or so. I recommend malwarebytes as it's proven useful in finding some hidden things and getting rid of them fully. I'd also suggest getting a hijack This! report and post the report here for some of the pros to look at and let you know what to remove.