On system startup you should only have programs that are necesary (maybe 2 or 3 at most) otherwise your PC is bound to take ages to load.
If you know your way around the registry you'l need to delete the entries which load up the programs at startup.
To do this go to: start > run > type in "regedit" > expand the "HKEY_CURRENT_USER" tab > Expand "software" tab > Expand "Microsoft" tab > Expand "Windows tab" > Expand "current version" tab > click on "run" you should see a list of startup programs there. For the unnecesary ones right click and click delete. Warning: Don't delete anything you don't understand what it is.
Defragmenting the drive will help, although its a time consuming process. Run an antivirus scan and make sure you have the latest definitions, and run an adware scan also, making sure you also have the lastest updates likewise.
If the slowness still persists being unbearable It may be a time to reformat, and reinstall to clear out all the rubbish you have on your pc.
Peace.