It may sound like a less efficient way of fixing an uncooperative PC, but it will give you a much better idea of what's actually going on inside and should make you aware much sooner the next time things start to slide.
1 One of the most obvious problems that can slow down Windows performance is having too many programs running at the same time. Try switching off applications using the System configuration utility. Go to Start-Run and type msconfig. Then hit "Enter".
A sluggish system is being hindered by something, whether it's simply laden down with far too heavy a burden of assorted files, held back by the onslaught of multiple autostarting applications or having to chase files and applications from pillar to post across a multitude of drives. If Windows is juddering along but continues to operate, it probably needs an overhaul rather than a full operating system makeover.
Finding out what's at the root of the problem can be a lengthy process, but it's worth going through and tracking down the cause.
Some things are easily fixed, such as adding a touch more RAM or archiving old files to DVD. In other cases, rogue files may be clogging up the system or conflicting with different programs. At this point, a deep breath may be required before you start a systematic inventory process in which anything that can't be readily accounted for gets the old heave-ho.