A system slows down for many reasons.
Like anything else, the more that it is used...the less efficient it becomes...just as we do
. The changes may be imperceptible, they may clearly be perceptible.
With older systemx, the typical way that I would approach this would be a periodic clean install, if I could not trace such slowness to any other factors.
It has long been a maxim of computing...that installing the maximum amount of RAM that Windows and the motherboard support...is the best way of attempting to gain max operational speed, all other factors not being considered. A system which accommodates 2GB of RAM will be faster with 2GB...than a system with 1GB.
Athlon XP systems are rather old by today's standards...when I had one, I employed 2GB of RAM, which was what my board supported.
I don't understand your reference to scripts slowing a system down. If you mean that you think that your Internet usage and storage of webpages is slowing your system down...then I suggest that you periodically dump your IE cache.
Any of the links at http://www.google.com/#hl=en&xhr=t&q=why+you+should+dump+your+browser+cache&cp=36&pf=p&sclient=psy&source=hp&aq=0n&aqi=q-n1&aql=&oq=why+you+should+dump+your+browser+cac&pbx=1&bav=on.2,or.r_gc.r_pw.&fp=8a58e93555963d21&biw=1440&bih=671
cover this in depth.
FWIW: Slow Computer-browser Check Here First; It May Not Be Malware - http://www.bleepingcomputer.com/forums/topic87058.html