You have to find out why it is slow. There can be many reasons for that, like:
- the PC has not enough RAM for what you want to do with it
- malware is running
- you run background software yourself (like SETI software, P2P, or other resource sharing)
- your antivirus or anti-malware is checking the hard disk
- an automatic Windows update is running
- Windows search indexer is indexing the hard disk
- it's not Windows that is slow, but only the Internet connection
- there are problems with the hard disk
- you run disk intensive programs on a 5400 rpm disk
- you're running CPU intensive programs on a slow CPU
- you're running GPU intensive programs (games) on a slow GPU
- it's only the boot that is slow, but once that is done, it's fast again
Depending on the cause, do something about it.
A solution that works quite often is to go back to factory conditions. if there are no hardware issues, it's as fast then as it was. On an old PC, it can take to few days to do all Windows updates again, During that few days it's slow.