Put in 2x512Mb RAM and you should be able to run everything at a more or less acceptable speed (although you'll notice it still struggles sometimes). I had exactly the same problem - 512 just couldn't cope. When I went to 1Gb the improvement was dramatic. The advice always is : put in as much memory as your machine will take.
Also post your version of security centre please
McAfee Security Centre V10.5 Build 10.5.237 Affld 571 L/Update 11/05/11
VirusScan V14.5 Build 18.104.22.168 L/Update 21/05/2011 Datu 6352 creation 20/05 2011 Engine 5400.1158
Firewall V11.5 Build 11.5.141 L/Update 16/12/2010
SiteAdvisor V3.3 Build 22.214.171.124 Affld 571 L/Update 29/04/2011
AntiSpam V11.5 Build 126.96.36.199 L/Update 11/05/2011 Engine 188.8.131.5249
Parental Controls V12.5 Build 12.5.113 L/Update 16/12/2010 (do not use this or SiteAdvisor.
All the above is free with O2
According to HP my computer will take a max of 1GB 2*512mb approx £36 looking on the John Lewis website they have a Compaq £279 with 3GB and one at £309 with 4GB plus other makes.
I take your point even with 1GB yours can still struggle mine would be the same I think, I am not sure if adding more ram is only putting of the enevitable. If McAfee keep wanting more of my limited resources it could come sooner than I would like.
I have a 2004 HP Pavillion 2.66 Celeron 512MB ram I get McAfee free with my ISP. A few years ago I had a problem and I used another paid for piece of software to cure that problem I was probably on version 2 then 3&4 came along ran OK V5 was slow and V6 brought the computer to it's knees, I contacted the company and the asked for the spec of my machine, the reply was it was not powerful engough to run the latest upgragdes return to V4.
I think I am hitting the same problem with McAfee mcshield mem 93,048 vm 167,420 in task manager every thing that seems to be McAfee related is using more and more resources. A couple of weeks ago PF usage hit 1.1GB while installing McAfee upgrades. Also had to go back to Firefox 3.6 from 4.0 that also seems to want more memory.
According to HP my computer will take a max of 1GB memory in two 512 sticks Is it time to invest in a new computer.
You can continue to run your computer and add another stick of memory.
If you are knowledgeable about computers, you might wish to assemble your own computer. from new computers parts and get exactly what you want.
You can also get a new Barebones Computer and build up upon it.
If you are not knowledgeable about computers, then buy a Brand Name Computer, like a Dell, Acer, HP, ASUS among others.
Be happy, but be warned, new Computers can have defective components. even after assembly.