I have a Dell studio 1537. When I remove my battery and use it with the power cord plugged in, it becomes very slow, even the startup slows down.
I would say it's down to the laptops' power settings. First I would check the BIOS of your Dell. Don't have one at the moment, but AFAIR you can access it by pressing F2 during the boot-up, but before the Windows logo.
Navigate to "Power Management" and change settings for both battery and ac power to "Disabled". To save up even more on booting time, head to the "Performance Options" where you should find "Hard Drive Noise Level". Check if it's set by any chance to "Quiet". For faster boot times you will need to change it to "Performance".
If you don't have those options listed in your BIOS, it's time to flash it to ver. A9 - the most recent BIOS update for your Dell Laptop.
If you like to put your laptop into sleep mode and experience any problems with waking up (unfortunately, it seems to be plaguing your Dell model) use ver. A6 or A7 instead
Among other things I would check Windows power settings as well. You will find them after pressing Start -> Control Panel -> Power Options. (If you have category view turned on, just change "View by:" option in upper right corner.
If you're using Windows XP option "Home/Office Desktop" should be yours. Under Vista and Win7 you need to click on "change plan settings" for active power plan and set it somewhere among those lines to make sure you are getting all processing cycles your CPU can provide.
Of course my laptop is running 24/7 so you may choose little more liberal settings for display and harddrive activity ;)
Hope that solves your problem!