Have you kept up with your housework?
Defrag and tidy up unused crap from desktop,
and how full is your hard drive?
is the inside of your case actually clean and dust free?
no buildup of fluff on heatsink/fans?
all these things and more can help get some speed back,
a new system would be better obviously, but I appreciate that may not be an option, and wasn't the question
lets see what we can do for this one before writing it off
If there is a lot of dust build up in the old system, it could be getting too hot and throttling itself down. Have you taken the time to blow out the case and heatsink thoroughly? Also, make sure all the fans are working. On an older system like this, fans failing would be a big possibility.