WHat anti-virus are you using? what do you prefer

[tchanabc]

bitdefender is good but it kills cpu

[WhoIsYou]

bitdefender is good but it kills cpu

No..
I have it, just checked all it's processes in task manager, combines it's using about 1% cpu and about 9-10mb of RAM... Which is nothing especially considering it gradually checks all your files really slowly while it's on the in the background, and it's watching the network at the same time + actively scanning any activity taking place (registry changes, attempted installations, any running programs or programs trying to run, etc).
It's also highly configurable to enable/disable any of those features that it has if you find they're using too much cpu/memory.
Of course, it uses a little more juice when an actual scan is going, but just do it over night if you must... Or just do quick scans which don't usually take all too long, although I prefer deep scans.