Would like to ask which Antivirus best for my situation-

5-10 office PC
OS: window xp & 7
hardware: range from pentium 4 to g630
sharing enviroment (printer, file)
one of the desktop act as server that has database system.

as some review bit denfender it good.
but for my low hardware spec PC, will it slowdown?
What if some PC unable to online, how are they going to update defination