How to boost performance and antivirus help!

I'm afraid the only way to have a performance boost is by installing an internal SSD, what seems to be not so easy.

Or you could connect it with an adapter to the Firewire 800 port, it's at least faster than USB 2.0, but you will not get near the full SSD speed since it is much slower than USB 3.0 or even Thunderbolt. But it may be a little faster than the internal HDD.

It seems your iMac has a 1 GB and a 2 GB RAM module installed. You could upgrade it to 6 GB maximum by just replacing the 1 GB with a 4 GB of the same type.

El Capitan will not speed it up, but it might also not slow it down. You could try it on a second partition.

I gave an early 2008 MacBook Pro to my Mum, it has just 4 MB and a HDD. El Capitan takes long to boot (don't know if SL would be faster), but after that it seems pretty good usable if you don't open too many applications at the same time and use it just for simple things.