A free Visual Basic compiler? This is not like C, where you have lots of possible compilers - Microsoft Visual Basic IS the compiler. If you already have MS Office, you can use VBA (ie Visual Basic for Applications), which includes parts of the real VB's features. Another free alternative might be the WSH (Windows Scripting Host) which also supports something similar to VB.
But all those don't compare to the full program, if you really want to program in VB, you'll have to buy it - if you're a student, try getting the rather cheap student edition, that's what I did.
BTW, Delphi is a language similar to VB (although where VB is based upon er Basic, Delphi's fundament is Pascal), you might also want to try that.
Keep in mind though that neither Delphi nor VB really compare to what you have on your TI ... both are incredibly more powerful.