I thought you were referring to a different area (later in the game) when you said lava fields.
If you can borrow the older computer long enough to clear the initial island, I don't recall a lot of other areas in the act 2 main game which mix mutants with other opponents.
I don't use XP that much, but you should get a warning when trying to shut down anything essential. Many programs that add themselves to the task bar beside the clock don't need to be running all of the time. With laptops there will be power/network/etc utilities always running, but things like media players or office suite search/quicklaunch programs are unnecessary.
A quick web search produced:
How to free up Windows XP system resources. Following the links there or another search may come up with better resources, if Spybot's startup manager isn't clear enough (I haven't tried it).
You may also want to play with the game settings, or check out suggestions in other topics, such as;
Awful Slowdown.
