I have a Acer laptop with a 1.5Ghz Intel Celeron Processor with 256MB of ram, and it lags as h*ll. But now its stable, I find that malwarebytes is a little demanding in terms of resources on my laptop, but its a very useful program and i do recommend running it to check for malware every now and then. I would suggest a ram upgrade, 128mb of ram isnt sufficient enough.
Just a little insight to my laptop, may help you in your case aswell. I'm using ESET NOD32 antivirus, very light on the system, and thats the heaviest program that runs on startup :).
You can look at this from two different angles, if the programs you are running now, either on startup, or manual lanch once everything is loaded, is absolutely necessary, then you can consider a RAM upgrade. Or you will have to sacrifice the use of some program so your 128MB ram stick can cope with the demand.