Ok found your hardware .... looks like a Pentium D 3Ghz with 1GB RAM... unless you upgraded to 2GB.
http://support.hp.com/us-en/product/Compaq-Presario-SR1800-Desktop-PC-series/1841793/model/3197020/document/c00708248/How much RAM do you have?
Are you still running the Pentium D 3Ghz CPU or did you upgrade to a faster CPU?
Have you performed a clean install yet or have you been just picking away at an old install trying to make it more efficient?
I see that your at SP2 and not SP3, if this is use for online purchases, banking, etc I would update it to SP3 with all security patches. There is a slight performance hit when being fully patched. I ran a benchmark once going from SP0 to SP1a then SP2 and then SP3 and the best for performance and driver and software benefits is SP2, however on a Pentium D I dont think the performance hit from fully patched to SP3 would be that bad. I had a Pentium III 600Mhz laptop with 384MB RAM that i went through the updates with and the problem was that SP3 required more system RAM mainly and so at 384MB RAM it only had like 16MB free whereas with SP2 i had almost 100MB free memory. Also the CPU was weighted down with SP3 on the Pentium III 600Mhz and so it was laggy, so I ended up bringing it back to SP2 and leaving it at SP2 for best performance & software support that required .Net and SP2 etc.
If this system is not used for anything critical you could keep running it on SP2, however if your using it as your main computer with banking and purchases online etc, i'd suggest SP3 patched fully. * You dont have to worry about optional driver updates, just the critical security updates.
The firewall that is built into Windows is usually plenty, especially when installed behind a router that has a hardware firewall of its own. I wouldnt go with a stronger firewall unless you have your system on the DMZ exposed to the internet such as port forwarding etc.
I would limit the system to only running 1 Antivirus, and use a tool like Malwarebytes to check the system say weekly etc to make sure all is well.
A clean install should be all you need. You could go further after clean install and remove some services that may not be needed, but the Pentium D 3Ghz should be plenty. Lastly having 2GB RAM instead of just 1GB could also help your performance of your system as for their would be less need for the system to page to HDD virtual memory info to disk.
And I have also seen systems run better when even a lower end better video card is used that is better than the integrated video that came with the system, especially for games or applications that default to Software Rendering when a weak GPU is detected which weighs down the CPU having to perform the duties of a GPU.
