If you were married as of 12/31/2007, you could not file as single on your return. You would need to file "married filing jointly (MFJ)" or "married filing separately (MFS)". Usually MFJ will have the better tax advantages, but MFS keeps your tax liabilities and refunds separate. If you do file MFJ, you should also fill out and file Form 8857 (innocent spouse relief) with your return to protect your allocation of the refund.
You may file jointly, however, if you do you should also file Form 8857 as described above.
I apologize as I gave you the wrong form number. You will want to file Form 8379 (Injured Spouse) to protect your portion of the refund.