Microsoft Security Essentials
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Full user review
Free for home use.
Made by Microsoft.
Does not interfere with the OS which other security suites such McAfee do from time to time.
Quite lightweight on system resources.
XP 32 bit, Vista/7 32 bit/64 bit compatible.
Integrates with Windows Firewall. Therefore the two Microsoft products complement each other.
In XP the firewall is relatively weak. In 7 and Vista the firewall has undergone significant enhancements. If using XP get a decent firewall.
As mentioned for Windows XP use in conjunction to a stronger firewall for 7 and Vista the firewall is okay but you may want to use a slightly stronger one. I recommend Windows 7 Firewall Control (there is a free version of it for XP and Vista also). http://download.cnet.com/Windows-7-Firewall-Control/3000-10435_4-10618117.html
I have used Microsoft Security Essentials in a range of different hardware.
A 8 year old IBM machine with 512 MB of RAM, P4 processor and Windows XP (need it as has special software and PCI cards on it). McAfee crippled it beyond feasible use. With security essentials it ran fine.
I have on the other end of the spectrum a brand new Dell XPS 8300 Windows 7 64 bit, 12.0 GB of RAM, i7-2600 CPU and 1.0 GB graphics card,naturally ran fine on that.
In conjunction to Windows 7 Firewall Control this leads to a pretty strong, stable configuration. Finally a periodic scan with the Free version of Malwarebytes' Anti-Malware is also recommended. I use these 3 products together and have never had any my machines brought to its knees. Note for sufficiently powerful machines (>3 GB RAM, > 1.8 GHz Dual Core CPU) I recommend getting the pro version of Malwarebytes' which offers realtime protection and using this in conjunction with security essentials. I have never experienced any conflict in doing so.