Painfully slow laptop - no obvious reason

Hello, I have an Acer Aspire 3000 laptop running Windows XP that runs painfully slow - not all the time, but at least 75% of the time. 5 minute waits for a window to open is not uncommon. I have run a full virus scan (Kaspersky 2010) - 13 hours later and nothing found. I have defragmented the C: drive (Piriform Defraggler) - 10 hours later there's no improvement. I have run Autoruns and found nothing suspicious. It just seems like it's always doing something else, something I can't see.

The hard drive is divided into 2 partitions, each a little over 17 GB. C: is 80% full (about 3 GB free) and D: is mostly empty. I have moved files over to D: and deleted unnecessary files wherever possible, to no effect.

I have run out of ideas here, and I would greatly appreciate any help or ideas. Thanks in advance

Slowness and performance issues as you describe are not uncommon, especially with older computers. Since we're not finding any malware, please refer to Slow Computer/Browser? Check here first; it may not be malware. There are reasons for slowness and poor performance besides malware - i.e. disk fragmentation, disk errors, corrupt system files, too many startup programs, unnecessary services running, too many browser Add-ons/toolbars, failure to clear browser cache, not enough RAM, dirty hardware components, etc. As you use your system it becomes filled with more files/programs and has a natural tendency to slow down so cleaning and regular maintenance is essential. Incompatible browser extensions and add-ons can impact system performance and cause compatibility issues such as application hangs (freezing).

You indicate that you have tried some of these suggestions but there are more in that topic.