Why my PC windows 7 takes such a long time to boot up?

Start scan quick scan. Remove all the infections and reboot the PC. Again launch the scan whether it is quick scan or full system scan.
Make sure that your PC is virus free. If you installed any antivirus, remove and reinstall the antivirus once PC is clean.

Reduce the startup items:
Start > Run > type msconfig > Startup tab > click Disable All button
click OK.
Then remove all temporary files from this location:
Start > Run > type %temp%
Start > Run > type temp