first has anything changed in the basic build?
have you installed any new software?
as vhg says only one virus scanner
ad aware is NOT a virus scanner ... so you can't use it instead ... you use it allongside the virus scanner
you should also have only one firewall
next question is ... business machine ... third floor
is that you are on the third floor ... or your company has three floors?
if you are using a corporate machine ... get your IT bloke to fix it - they will know the build and software ... you could create a real mess by using diy solutions (where I work it's a sackable offence~~) - most "free software is for home use only - your company won't appreciate you putting them at risk
if not ... it sounds like the machine is just getting full
your c: should have AT LEAST 20% free space - aim for 50
install and run
http://www.ccleaner.com/download
back up as much as you possibly can (at least once)
and remove
uninstall progs you aren't going to run - just in case is dumm when IS slow is reality
run scandsk
and then defrag
and that's about the best you can do
more radical - reload windows
fit more ram as a last resort ... if it was OK but has slowed down without any big changes more ram isn't going to do much
and finally
buy a new machine