Ultra Slow Boot

From turning on the computer until the hard drive stops reading is taking around 30min!!!!!!!!!!!!!!!!!! It always seemed to slow ( 5-8min) but all of a sudden the boot up time has drastically increased. I have upgraded and installed and uninstalled so many things recently I'm not sure which one could have done anything.
I have Defragged using standard windows application and also using 'Diskeeper'. This made no difference. I have deep scanned the whole computer for addware, spyware, viruses etc using AVG, ZoneAlarm Pro, Microsofts onilne scan, Dells support scan and several others, all of which show nothing is wrong.
Looking online at others with the same symptoms, it seemed the Zonealarm was using a lot of memory power so I uninstalled this but no luck changing the problem.
Once the boot up has finished ( anything from 20-40 min ) the computer runs fine with no problems at all.
The Windows task manager ( when it eventually loads during the boot up ) shows the PF Usage at up to 1.4GB and the CPU fluctuating from 0% up to 50-60%.
I've also had 1 warning about virtual memory being high.
Any help REALLY REALLY appreciated!!!!!!!!!!!!!!!!!!!