|So you seem to be saying that you get the message about the drive not being found then the 'press F1 to continue or F2 to enter setup' or words to that effect. So you press F1 and it goes ahead and boots into windows?|
If that's the case you must have a SATA drive specified in bios setup that's not there and windows is on another drive that is there.
I'm not familiar with every piddly-ass bios out there and I don't know what hardware you have or how its connected so I can only give general advice. That advice is to go into bios setup and make sure the drive(s) are configured properly. You may need the manual for that. If you don't have it it should be available for download on the Dell support page for your model.
As I mentioned, there's usually a bios 'default setting' you can enable that will choose the settings most likely to work. That may be all you need to do.
One thing that can cause bios setup to lose its settings is if the motherboard battery is weak. When you get into windows check the date and time. If they're way off it's probably the battery. When that happens you change the battery and then go into bios setup and configure the lost settings.