Don't think there is a single answer, but I'd first try and figure out the bottleneck. Some are easily corrected, others may not be. For starters I'd do the easy ones. Add RAM, especially if one has low RAM like 4GB's or even 8GB's. Second, swap out the boot and primary hard disk for an SSD and copy over the original drive. Also look at your STARTUP program lists (in Windows use the StartUp tab in Task Manager) and disable ones you do not need. Internet could be slowing you down, consider replacing your router if it can't handle the speed of your ISP provided Internet. If you Network adapter is slower than your ISP speed, consider replacing it or using a higher 802.11 rating speed USB Network device if wireless.
What are your best tips for fixing a slow system? For more questions regarding this contest, please visit the Information Page here.
DS 07/23: What are your best tips for fixing a slow system?