OpenDNS
USED to block Malware domains for free, but not anymore
Now to get full protection you gotta cash out a bit (10$/year)
Still it's worth it, especially on such low ram (512mb)
On side note - Use Google Chrome, OpenDNS and don't open email attachments with executable files and you'd be 99% safe. If I ever get programs/games of questionable nature
I use Virustotal.com to scan suspicious files (it scans using 43 different AVs)
I'd say raw speed is more important than false feeling of safety security AV vendors trying to give you...
Windows 7 is big step in right direction in terms of security and speed. I have old IBM x31 laptop with 1gb of ram and since I switched it from xp to 7 - I felt the performance was improved - not light and day, but improvements were clear.