PC is lagging

Ok, let me tell you what I know slows pc's down...spyware, viruses, temp
files, junk like that, registry problems and almost full HD.
I have deleted 1 of 4 user accounts to free more space. used clean up and
ccleaner, used ad-aware and AVG, used registry editor, etc.

my 8 yr old Dell has a sad 40 GB HD and the ram is 256 MB (nothing to jump
about). It runs XP home and the processor is 1.79 ghz with pentium 4.

I recently replaced a power supply. I rarely use this desktop becuase i use
my laptop. But, today when i turned it on it took 8 min to get out of
welcome screen and just as long to load desktop. and everything else
dragged. I restarted it and its doing better. but still lagging a bit.
BTW, I made sure that i only have necessary items at start up (msconfig).

So is it possible to replace the 1.79 GHZ processor with a higher one. Or is
it something that comes already on the pc and cant be changed.

Knowing what you know, what else can I do to help the pc function better. I
have high speed internet so even that was slow; but the internet is not the
prob....its the pc. I still have more than 50% free HD space.
Thanks in advance for your time