@prathap_lab, i also own a HP Pavilion, (256+512MB RAM),
What i've Installed is====>>

Zone Alarm Pro (Only FireWall- Paid version)

AVG Free Anti Virus (but its not in Start-up-> its my BackUp AV)

Kaspersky Internet Security 6 (Full Version)

Windows Defender
Now if you dont have a paid ZoneAlarm Pro, then your best bet is Kaspersky Internet Security as it has FireWall.
(i did not install the Kaspersky FireWall as i have the Best FireWall already installed)
The PC starts up as usual, in 1-2 mins. even when i had ZoneAlarm+AVG Free+Windows Defender, when my PC had only 256MB RAM config.

All these programs take an average RAM of 5MB, so your PC will not Crawl.