Real-time protection that slows system least?

I have an older desktop computer that I don't use much anymore. It has 1300 MHz CPU, 500 MB of ram (max), and has Windows XP Pro with SP3 and up-to-date updates. I also tun Online Armor Free on it, and at the moment Bitdefender Free, but I'm looking to switch the latter to something offering real-time protection at minimal resource use, especially in terms of RAM since 500 MB is nothing these days...

I had AVG 7.5 on it, but version 8 was too resource hungry. Rising AV is also heavier than I want. I'm leaning towards Avira AntiVir Free, but haven't tried it, so not sure if that's too intense as well. Naturally, I'm not expecting the level of protection offered by AVG 8 or similar, and I will mostly rely on Acronis backups and manual scans (a2-squared cmd line, and various freeware anti-malware scans that I run portably or sandboxed i.e. without permanent install).

What would be good? Is Avira AV Free resource hungry? What about Avast?

P.S. Bitdefender 10 Free is not too bad, but I don't like the half dozen always-on auto-start processes it added to my system, plus I'm convinced it has slowed Windows startup.

 

The Comodo option looks interesting. How does the FW compare to Online Armor?

Will it protect against attacks from file-sharing networks (emule)?

Does it ask before letting new programs call home or otherwise use the internet?

Does it play fine with home network softwares and setups? (fwiw, I use Teamviewer and Synergy at the moment, plus a USB-to-USB network cable (requires NetBEUI), and a shared drive (on the home network) so that I can use my laptop's mouse and keyboard to control both the laptop and desktop, and copy, paste, transfer files, etc. between the two. it's just easier that way because I'm always on my laptop anyway, and the (old) desktop is now just for side projects.)

 

update: I pulled the trigger on CIS (Comodo Internet Security). It's only been 2 days, initial impressions are positive (with default settings).
-basic install was quick, but the followup update and scan took several hours
-initial CPU usage was high; CIS maxed out resources for 20-30 min after initial reboot, but the system didn't freeze - it must have used only spare CPU, and then it settled down
-currently CIS is performing flawlessly to block and warn about threats without any noticeable slowdown in my computer's overall performance
-CIS asked me to approve the pings or id requests from my other computers, the connection from my USB-to-USB network connection cable, synergy (used to share mouse and keyboard), emule (file sharing), and other applications I expect to be notified of.
-my eMule user Id went from High to Low though, so I do need to tweak with that, but at least I know its blocking all the usual attacks through that door.
-CIS also detected a trojan on a file I copied over the USB-USB cable (well, a-squared on my other computer also reported a trojan, but Rising Anti-Virus did not - but Rising is less strict and often waits until a malicious file is executed before blocking it).
-initial scan by CIS produced several dozen false positives, which while unfortunate tends to be a necessary evil. the FPs were mostly for standalone apps I downloaded from MG or other trusted sources; apps by SysInternals, NirSoft, DiamondCS ConsoleTools, FloppyOffice, Windows password/product info recovery tools, etc.
-CIS has played nice with another anti-Spyware app I'm testing, Rising PC Doctor, although it did ask me to green light the process and call home requests from that program.

as for ONLINE ARMOR and BITDEFENDER, I disabled all their autostart processes and services, but have not tried to uninstall them yet...

RISING PC DOCTOR: http://www.rising-global.com/products/rising-pc-doctor.html

I added this after disabling BitDefender, but before I disabled Online Armor and installed CIS. I decided to keep this running because it's somewhat unique and because it was advertised as suitable for use with other anti-malware programs.

The program is free and easy on resources, except maybe on your internet connection as it calls home a lot. I'm guessing it works by updating and referencing an online database of all user statistics to evaluate processes, services, and other info. i.e. if you trust a particular process, you can mark it as trusted, and I suspect that if a lot of users mark a particular process trusted, then PC Doctor tells you the process is known and likely safe.

It has other nice features too. For example, it informed me I had the guest user account enabled with no password, even though Windows XP displayed it as disabled under Control Panel -> User Accounts. I had to use the run command 'control userpasswords2' to see the real info.

Also, it alerted me to 'missing' hotfixes that were available for download from Microsoft, but that I hadn't installed previously because they weren't shown in regular Windows updates.

Negatives: Have to kill PC Doctor process from process manager or similar to end it completely. Instructions for fixes may be slightly inaccurate. Some 'fixes' may 'fix' more than you bargained for. For example, when I disabled the guest account, my laptop lost access to the other computer's shared partition (but at least I know the issue now so I can put a password on it).