I think there are three major indications for a hard drive problem:
1 System freezes when coming out of sleep mode
2 Playing particular games (which involves huge amount of data transfer to and from hard disk drive)
3 Freezing while disk check (and at the same cluster number)
You also mentioned about sluggish performance which can also be caused due to some software. But after disabling the startup programs from msconfig, we can eliminate the software related cause.
I suggest you to take a backup of important data immediately and contact a service center.
You can try the following link for warranty information:Link
You can also call the following number for Dell Support
Technical Support 1-800-624-9896
Customer Service 1-800-624-9897
I am sorry that I could not provide much help with this problem.