Paul,
here’s my take on this.
Disclaimer: I’m not a system builder, but a musician. My experience with DAWs stems solely from researching and building my own music computer. And I know very little of you and your setup, that’s why I assume a whole lot, and I will make some general observations about DAWs. Don’t be offended by anything I’m saying… 
I use two computers as well, one for office work and one for production, former is an 8 year old Core 2 Duo laptop and the latter is in my sig.
Here we go: You have two laptops and need to decide what to do with them. The 3737 machine is newer and more powerful and could serve for production and you could use the 5040 for office stuff and whatnot.
Unfortunately, I cannot help you with your error messages and the system running poorly, but here’s what I would do, and it’s only partly “throwing money at the problem”, because this will clear the slate and make the system(s) predictable, because you know precisely what went in and what you can expect to get out.
- Get a barebones Windows 8.1 OEM x64 systems builders DVD. If the DVD supplied with the 3737 is clean, use it, otherwise forget it and use the clean new one, since you don’t want to install possible bloatware again.
- Get two modern SATA3 SSDs, I use 850 EVOs and love them. Maybe one 500G and one 250G.
- Get a 2TB 7200 rpm SATA3 HDD, since I assume you want to record all those cool keyboards you have…
- Get a SATA3 to USB3 cable.
DAWs benefit tremendously from discrete storage for at least audio and Samples, and the OS needs a fast drive for booting and system tasks.
Clone the existing drive in the 5040 to the 500G SSD and install it into that laptop. I did that to my ancient laptop and expect to be able to use it until I die. NO issues whatsoever.
Or: Even better would be to install another x64 Win8.1 OEM onto the 5040 as well, so that it can have a new life and the same look & feel as the other computer.
Again, I did this and no issues, except for finding a graphics driver for the Intel graphics from 2007…
You won’t recognize that computer, promise!
Then: Put the 250G SSD into the 3737. Go to the BIOS and disable all sleep states and anything related to power savings, e.g. Intel SpeedStep (EIST). On a Dell, there might be little to tinker with, but try anyway! Disable the internal audio and all networking that you don’t absolutely need. Put SATA ports in AHCI mode. You might be able to reuse the HDD from the 3737, but if it’s an 5400 rpm drive, toss it… 
Install your fresh copy of Win8.1. Update the system completely though MS update. Do not install a virus scanner, Win Defender is fine.
Install the new internal HDD, this will become your new audio drive.
Use the old HDD from the 5040 (or the 3737, if you didn’t toss it… 
) with the USB3 cable as your external sample storage, or use an existing external drive. Sample storage doesn’t need to be really fast, if you don’t mind waiting a little for projects to load. Some VSTis use disk streaming (Toontrack for example does not!), so for Superior Drummer it doesn’t matter, for others, like Ivory, you might lose a few voices.
Do not install ANY manufacturer supplied drivers, only if Windows update can’t find them.
Then install the audio/MIDI interfaces’ drivers, then Cubase, then all your VSTi. Then the other music software.
Run Cubase in between each step and see how it behaves…
Now you have a fast Office computer and a nice DAW with three discrete drives and a bunch of RAM.
Sorry I can’t help you with your existing problems, but I wager that you’ll have much more fun after this is done. More music, less troubleshooting.
Let me hear your thoughts, please!
Sincerely,
Benji