Judging by their overall WorldBench 6 scores, none of the PCs performed notably better after we ran the utilities. In fact, only two systems saw score increases at all after we ran a cleanup utility, and those increases were only a single point (CCleaner, System Speedup, and WinOptimizer 7 each produced a one-point bump on the Dell E1505, and WinOptimizer 7 did so on the Lenovo ThinkPad Edge). Most of the tests produced scores that were either identical to or slightly lower than that of each PC's original configuration--not quite what you'd expect from performance-boosting utilities, considering that a one-point gain in WorldBench 6 is small enough to be within the margin of error.
However, the test results for individual applications yielded a few more-interesting tidbits.
360Amigo's System Speedup seemed to have a problem with Microsoft Office. For each of the test PCs except the Dell E1505, Office performance grew noticeably worse--over twice the test time, in some cases--after we ran System Speedup. This issue with Office accounts for the dramatic drop in some of the WorldBench 6 scores. Aside from the Office test, though, System Speedup's results weren't that different from the others.
Some of our PCs were more heavily affected by cleanup utilities than others. For the Toshiba M645-S4055, performance didn't change by more than 10 or 15 seconds in any given WorldBench 6 test--not surprising, considering that it was carrying the most powerful hardware. The Dell E1505, with the poorest specs of the bunch, saw more-dramatic differences, both good and bad, in specific tests with each cleaning utility (photo editing ran about 10% faster, creating and saving a DVD image ran about 20% faster, and video editing in VideoWave ran about 5% slower). So even though we didn't find substantial differences overall, your older PC might have a particular problem that one of these utilities can fix.
There are no one-size-fits-all PC fixes. After running each utility, the Dell E1505 saw a big speed improvement with disk-intensive tasks such as writing DVD images. In contrast, the Labs' custom-built desktop did worse in that respect: An 8-minute test took about 20 seconds longer after running CCleaner, System Speedup, and WinOptimizer 7; and it took a whole minute longer after running System Mechanic 10. The Dell D520 had slightly slower speeds in that task as well. This seems to mean that even if a utility made one PC run better, it might not necessarily help yours--and it could even make your PC's performance slightly worse.
Although we ran each cleanup utility three times before running WorldBench 6, some utilities still found items that needed cleaning each time. We don't know what caused this; the utilities might have encountered problems they couldn't fix, or the cleanup process might have created other problems elsewhere.