On power-up the DEL or the F2 key take you into the BIOS or the F7 key can be used to go straight to a boot menu.
The ViewSonic SC-T45 BIOS identifies itself as Aptio Setup Utility and is copyright AMI. My unit was running version SCT45R01. Scanning the Linux dmesg output I see: DMI: ViewSonic Corp. SC-T45/SC-T45, BIOS R01 11/08/2012
Essentially what appears in the boot options depends on what the BIOS finds as the system powers up.
With a 1GB pen drive plugged in the F7 key brings up:
Please select boot device:Realtek PXE B01 D00 PATA PM:SLC4G USB2.0 Flashdisk 1100 UEFI: USB2.0 Flashdisk 1100 Enter Setup↑ and ↓ to move selection ENTER to select boot device ESC to boot using defaults
In the BIOS 'Boot' screen you get an identical list under 'Boot Option Priorities'.
According to the datasheet it would have been Windows Embedded 7 if the flash hadn't been wiped clean.
My experience here is that there aren't any - which is a pity as it appears to have a buggy ACPI BIOS.
As always I started out with Google and searched for 'ViewSonic SC-T45'. This turned up a lot of hits most of which linked to the ViewSonics.com website. Following those links just resulted in 404's - page not found. The search function on the ViewSonics home page also failed to turn up anything, but I persevered and, having navigated to https://www.viewsonic.com/uk/products/archive/ scrolled down past the all the monitors and projectors to find mention of four thin clients one of which was the SC-T45.
Unfortunately that didn't really help as the only download offered for the SC-T45 was for their device manager software.