Reinstalling via in-place upgrade of W7x64+SP1 is likely to be the easiest way to cure that problem assuming Dell is correct in it's diagnosis.
You could also try reinstalling the service pack itself first to see if that catches the gremlin. You can download the service pack directly from Microsoft here
Note: It's a 1GB download, so if your web connection isn't up to it I'm sure some fellow DCer could snail mail you a copy. If you're in the USA, drop me a PM with where to ship and I'll be happy to send you a disk.
re: 24Gb of RAM:
I'm not sure what you were thinking either! Wow!!!
FWIW, I doubt you'll ever come close to using anywhere near all of that. Probably not even half unless you set up something like a half dozen extremely robust virtual machines.
As far as monitoring how much is actually being used, any 64-bit capable memory monitoring utility should be able to keep you posted about what's going on under the hood. Windows built-in task manager will also give you memory usage and stats if you look under the performance tab.
I personally like the oddly named SysInfoMyWork
utility when I occasionally want to peek at how much memory and CPU is being used. Can't vouch for how well it works with a 24Gb machine since I've never had one however.