Possibly a bug or Trojan of some sorts??
wants it to keep running ... so it can spread the bad stuff more.. or even worse send your info out

I'd unplug off the internet as soon as possible after downloading a few extra scanners- and run a few deep scans with each one to clean out anything questionable.

moi likes the malwarebytes program - free to download & use
http://www.malwarebytes.org/

cnet.com editor picks for spyware removers -
http://download.cnet.com/windows/spy...g=ltcol;narrow

I think the non shut down is a symptom of a trojan or spyware , but her are some tips on improving shutdown times.

shut down ideas -
http://www.ehow.com/how_5002443_spee...down-time.html

found this too-
http://en.community.dell.com/forums/t/19265619.aspx
and more-
http://www.google.com/#hl=en&q=dell+...6c7832dbb01be6

but until you figure it out don't keep it connected to the internet full time, unplug or turn off modem when not using computer online.
just to be safe..