I'm gonna assume you 'borrowed' your script from a VBScript example. vbTab is not used in KiXtart. You can simply insert a 'tab' inside your quotes to tab over. It can also be used as a delimiter, for import into Excel, for example.

You can always 'assign' a tab to your $vbTab if you want...

code:
$vbTab = "	"
Also, I should warn you to use "+" when joining your output. Example...
code:
? "Size: " + $vbTab + $objDisk.Size
This is considered 'best practice.'