I am having the same issues with all eAthena clients, I can run an Aegis client just fine, but any eAthena client rapes my cpu.
I am running on 2gb of ram and it does that.
well i agree with panda, keep it clean ppl (desktop, registry, etc)... and eventhough some ppl say u should reinstall everything every 3 months, i prefer to use Norton Ghost... it's a nice and efficient backup system, and doesn't kill your CD like reinstalling would...
defragmenting it every month is also a good suggestion...