I moved from Tx to Va in September. I never switched companies though so I was only sent one W-2 form, and only entered the state of Va on that section. Do I need to do anything special for the online process?

I also got a 1099 form for the interest earned on my savings account, and I put Tx for the state on that one since my account has always been a Tx account.

I'm using taxslayer.com, does anyone know if this is a reliable site or if I should use a different one?

Your Sept to Dec, your income is VA income. Any interest received after Sept is VA interest.

If you are present in a state, then any income earned during this period, must be reported to the state. Read: Your U.S. Tax Return: Working or Living in Two or More states