History of the computer makes a huge difference in the answers you get.
like - when did it start running slow? - did you add programs or download files since it was running OK? -- what model is it and how old is it?
HOWEVER, the first thing I would do is to perform some maintenance.
1st - run [link|http://www.geekstogo.com/forum/files/file/187-tfc-temp-file-cleaner-by-oldtimer/|TFC|new_window=true] - download from here [link|http://www.geekstogo.com/forum/files/file/187-tfc-temp-file-cleaner-by-oldtimer/|http://www.geekstogo.com/forum/files/file/187-tfc-temp-file-cleaner-by-oldtimer/|new_window=true] and run from desktop = this cleans out temp files, caches, etc. Some malware might be residing there.
2nd - run CHKDSK = open a CMD prompt as admin and type ***CHKDSK C: /f*** = scans your HDD for errors. A major cause of slowness. The chkdsk command may take a very long time to run if there are errors.
When you run this with the /f switch, it will ask if you want to run it after a reboot. Answer yes and reboot. If you have other partitions/drives do the same for them (change drive letter). They won’t require a reboot.
If you just want to see if there is trouble on C: do not enter ***/f*** and it will run without a reboot and tell you if there were any errors. You then need to run with the ***/f*** switch to fix the errors.
3rd - run SFC = open a CMD prompt as admin type ***SFC /SCANNOW*** = this will fix any system file errors
Then run the following in the CMD prompt. You must be online for them to run.
***DISM /Online /Cleanup-Image /CheckHealth***
***DISM /Online /Cleanup-Image /ScanHealth***
***DISM /Online /Cleanup-Image /RestoreHealth***
All of these may take a long time, especially CHKDSK if it finds errors. Just wait for it. I have had it take all night to finish.At this point, test it. If it is still slow you need to run a complete AV scan.
After that, if it is still slow, boot in safe mode to see if it still runs slow. If not, then you have an app that is using a lot of resources.
Open Task Manager to see what it might be and close any that are using a lot. Some Anti-Virus programs use a lot of resources, especially when you are on line.
If it is still slow, a RAM upgrade might be needed. This would be indicated if it runs slow with your browser or other app.
Lastly, things change. You may have become accustom to a newer computer and are comparing yours to that. If so, you can upgrade to an SSD. That will significantly speed up your boot and lots of other things.