first of all your computer is slow, not the antivirus that are causing problems.
512 mg just doesn't cut it in 2013, even if you uninstall the antivirus, it will still be slow.
Also you have a celeron witch are a slow one core cpu, totally outdated and normal to be slow.
Please note that I plan to upgrade my Memory(RAM) from 512MB to 2GB.
You need at least 2 gb of memory if you want better performance, good thing your upgrading.
I have installed the 2 best antivirus, Bitdefender and Kaspersky,
second, NEVER EVER install 2 antivirus: they will conflict and slow down your computer even more.
Now about antivirus,
Performance wise (sntivirus not slowing down computer)
I recommend Microsoft Security Essentials (+1 fannatick, attal, ) But also Bitdefender and Kaspersky are pretty good antivirus (paid)
But norton and avg will make your pc slow down as hell (-1 danieladdison, J_E_D_70)
They are good antivirus, don't get me wrong but takes too much resources.