Did it just start like this suddenly or has it been getting increasingly slow over a period of time?
Does the slow period start prior to reaching Windows or after?
Does it run ok once it's booted?
Does your version of Dell include the Full System Diagnostic/ If so have run it, I believe it's accessed via F12 at boot up.
I guess you've already run all the usual checks... Anti-Virus, Malware, Spyware etc.
System Restore, back before it started.