It's only natural for computers to slow down over time. But that doesn't necessarily mean shelling out for a new machine. PC tune-up software promises help in pinpointing and removing the junk clogging up your hard drive. How well do the programs work? We tested four of them on a sluggish, three-year-old PC.

Auslogics BoostSpeed 5

Our top pick, BoostSpeed has a bare-bones interface but produced the best results of the bunch. After scanning your computer for space-hogging programs, it displays them according to size in a handy tree diagram. During our test, the software detected a whopping 35GB of wasted disk space. We also used it to tweak our browser settings to maximize downloading and Web surfing speed. The result? A dramatically faster PC. cost: $50

TuneUp Utilities 2011

TuneUp Utilities lets you scan your computer for system file problems, search for unneeded programs, and tweak Internet settings for added speed. A star-based user-rating system helps you determine if an application is worth keeping. We liked the software's intuitive interface, but it found only 5GB of wasted files during our test. After removing them, we noticed a minor uptick in speed. cost: $40 in stores and $50 online

SlimWare SlimCleaner 1.9

This software scans your computer for large program files and presents them in a list with ratings and reviews by other users. You can remove unneeded files with the press of a button. During our test, the software found only about 900MB of wasted disk space, and most of the programs on our PC were rated Good, which was not very helpful. After deleting some of them, we noticed a slight improvement in speed. cost: Free

IOBit Advanced SystemCare Pro 4

We were underwhelmed by this software, which pinpointed only 145MB of wasted disk space during our test, the lowest amount of the bunch. There was no noticeable improvement in speed after we deleted the suggested files. One upside: Like TuneUp Utilities, the software lets you schedule regular background scans. Like the other software here, it does not work on Macs. cost: $20 a year