I haven't actually tried this, but I believe you can't set up a RAID-1 set from inside Windows if it's the system disk. The issue is since you have to set the BIOS to boot from a specific disk, if that disk fails the BIOS won't boot from the other disk (at least until you manually change its configuration). But I could be wrong on that.
If you set the RAID-1 set using the motherboard BIOS / chipset configuration, then that problem goes away since the chipset BIOS is smart enough to be able to boot from either drive.
I'm going to disagree with sminlal based on what I've seen before, but I really suggest you ask Dell for their take on it.
On many systems with RAID1 support built into the mobo chipset, that system includes some RAID management software in BIOS. You won't see it until you turn on RAID capabilities in the BIOS and add a drive. But you might find all the details you need in manuals on the matter from Dell, so look for those. They could have come with your machine, or you may have to search their website and download.
On systems I've seen you create and manage RAID arrays using that built-in software, NOT Windows tools. In fact, some RAID1 management tools would allow you to convert a single stand-along HDD to a RAID1 array by adding a matching HDD unit and then having the system copy all the data to the second drive. This is almost the same as the process to repair a damaged RAID1 array by replacing one faulty drive with a new one and rebuilding the array.
HOWEVER, Windows ALWAYS needs a RAID driver installed in it to use any RAID array. If you are simply booting from a "normal" stand-alone disk into Windows and then using a RAID array as a data storage system, the RAID driver installation can be done any time after Windows is running as you set up the new storage system. BUT Windows in ANY version cannot BOOT from a RAID array unless the RAID driver has been installed at the very beginning of the OS installation. This implies that, if you want to convert your single disk into a RAID1 2-disk array and use ONLY that as the HDD resource in the machine, you will need to re-install Windows, this time including the required RAID driver install at the beginning, in order to boot from that array.
I have read that there may be ways to convert a non-bootable RAID array into bootable by some complicated editing of Windows registry and boot files in order to force it to load the right driver at the right time from the right spot on a HDD, etc. But I am not SURE that would work. It's definitely for the adventurous knowledgeable techie.