Avast didn't detected infections in the system.

I have Avast installed on my laptop (Core2Duo 2.2 GHz, 3GB, 320GB, Windows 7 Ultimate 64-bit) as well as desktop (AMD Athlon X2 5000+, 4GB, 500GB, Windows XP SP3) and I thought that my system was safe. Both the systems had Avast! installed, fully updated. I scan the systems regularly.
Yesterday, just to give it a try, I tried Bit Defender Online scan and to my horror it detected 12 infections in my desktop which Avast! didn’t. As Bit defender online scan doesn’t provide removal option with online scan, I tried ESET online scanner and it detected & removed the same 12 infections. I am using Kaspersky trial version now and feel safe.

Worried I tried ESET online scan on my laptop and it detected further 3 infections in the system. The infections read ‘Win32/ Agent.FGHQVIS trogan’. Why didn’t Avast! detected these infections? Is it because I am using a free version?

Although I am a student I don’t mind paying for an AV but as it is advertised that Avast! Home version is equally safe I didn’t bothered to go for a paid version. I am thinking of getting a paid version now but I don’t think it will be Avast!
I would like to thank Avast! team for the free AV.

Thanx.