Windows xp starts after 12 hrs

I would first be scaning the hard drive for errors... or you may have 9999 viruses and a scan with malwarebytes may be a good idea.

virus, old pc full with junk, hard drive failing .... ect. can be millions of reasons. you need to provide a better description.

some pointers:
windows version xp,vista,7 32/64bit
age of pc
do you have antivirus

Agree, lot more info needed. Can be sooooooooooo many reasons!

You sleep too much??? <grin>

format it and reinstall it. if it takes 12 hours to boot off a fresh install. throw it out the window... (make sure you dont hit any 1 on the way down  

Have you installed any windows updates recently or updated any software/downloaded anything that could have triggered this to start happening?

Open up command prompt and type "CHKDSK" and press enter. This will check your HDD for any errors.

If everything comes back ok then proceed to run:

1) Virus Scan
2) Disk Clean up
3) Disk Defragment

If all of these still don't resolve your issue, try a system restore (to a point before you made any software changes or downloaded any software updates).

If this STILL doesn't resolve the issue then formatting and re-installing your OS is an option, however its more likely by this point you just have a failed HDD.