Anti virus for old pc

[system]

Hi, I have a i7 laptop with avast and an old pc (from 2005). The laptop runs fine with avast but the problem is PC.
Ive upgraded the monitor and speakers of old pc so couldnt discard it now. Also, I was running avast 4 on it during 2008 it was slick back then…
Now avast has become bigger in size and specs and my Pentium 4 3.0ghz + 512 mb really feels the slowness when running latest avast… I mean even when u try to open “My computer”
I need a basic protection to PC and Im not sure what to do.
Microsoft security essentials is only 10mb in size and I have replaced avast with it for the time being. This way, i could use the PC for movies and some casual browsing…
Is there any light weight avast which i can use here… or would Malwarebytes (around 16mb) + MSE would give the basic protection needed ?..
I understand that we have lot of avast evangelists here but sorry my old pc cant handle avast free… Need an alternative…
Kindly help.

Almost all AV like avg kaspersky avira top 120mb+ size for installer itself and consume huge system resources. My only saviour was avast till now… Kindly help on how to keep the old pc running…

[Pondus]

What OS is on it?

[system]

Its on XP now, will be switching to tiny7 soon… as XP has ended…

[CraigB]

You upgraded the monitor and speakers yet you didn’t do the ram ??? 512mb is very low, stick 2gb or more if possible in it and it’ll amaze you

[Pondus]

Since i dont want to advertice for the competition here… i sendt the suggestion in a PM

[system]

well, the module supported is DDR… we cant use ddr2 and ddr3 in it… getting a ddr is tough these days, and even after that upgrade the pc is not gonna get much better… need graphics, multi core processor and list goes on… so just planning to not touch hardware and maintain old softwares and keep it running as a media center pc… MS office 2007, firefox…

the only prob is anti virus alone we need to have the latest versions

Thanks Pondus