Well, like others are saying you need to provide more info about your PC, and yes, the spyware, trojans, viruses, etc can slow down your PC. But to speed up your ole PC (and really fairly new PC's) check into some of the stuff that is in the Newbies or Computer Help Forum, like all the crap you can delete from your PC that will increase speed. Mod Grif can get you going with some of his stuff as well as Mod's Toni H. and Bob P.:
http://reviews.cnet.com/5208-6121-0.html?forumID=45&threadID=22053&messageID=238474
http://reviews.cnet.com/5208-6121-0.html?forumID=45&threadID=22053&messageID=243397
http://forum.aumha.org/viewtopic.php?t=7265&start=0&postdays=0&postorder=asc&highlight=
http://reviews.cnet.com/5208-6121-0.html?forumID=45&threadID=22053&messageID=238454
I don't have a Dell, but one of my older computers is a 500MHz, and have got it tuned up pretty good by doing all of the above including getting free anti-virus program, free Spyware programs, and free Firewall, installing more RAM up to 512MB, increasing the power supply, and a better Video Card. Maybe not quite as fast as my current main computer, but still good for excellent surfing (using browser Firefox kinda helps over IE) and play games other than the modern high end ones.
Watch the flyers from Best Buy, Circuit City, CompUSA, Home Depot, Office Max, etc for good sales on RAM. Goto Crucial.com to find out exactly what RAM your PC will take & the max.
BTW, great sale right now on an 80GB external Hard Drive 7200RPM at Circuit City for $39.99 after rebates. I bought the same thing couple months ago for $69.99 after rebates (regular $99.99) that I now received. Already formatted, simple to install/CD drives, USB 2.0 and regular 110V power connection, on/off switch, Backup Switch....if I wasn't saving my pennies for a planned trip to Vegas I'd get two more.