$655 million, and $389 million during 2008, 2007, and 2006. The increase in each year is primarily a result of increased revenue and gross profit relative to operating expenses. Additionally, 2008 includes the $53 million non-cash gain on the sale of our European DVD rental assets included in “Other operating expense (income), net.” 

Interest Expense and Income 

The primary component of our net interest expense is the interest we incur on our long-term debt instruments, including our 4.75% Convertible Subordinated Notes, which were no longer outstanding at December 31, 2008, and our 6.875% PEACS, which had a principal balance of €240 million ($335 million based on the exchange rate) at December 31, 2008. Interest expense was $71 million, $77 million, and $78 million, in 2008, 2007, and 2006 with the decline primarily relating to principal reductions of $899 million in 2008 and $300 million in 2006. 

Our total debt was $468 million and $1.3 billion at December 31, 2008 and 2007. See Item 8 of Part II, “Financial Statements and Supplementary Data—Note 5—Long-Term Debt.” 

Our interest income was $83 million, $90 million, and $59 million during 2008, 2007, and 2006. We generally invest our excess cash in investment grade short- to intermediate-term fixed income securities and AAA-rated money market funds. Our interest income corresponds with the average balance of invested funds and the prevailing rates we are earning on them, which vary depending on the geographies and currencies in which they are invested. 

Income Taxes 

We recorded a provision for income taxes of $247 million, $184 million, and $187 million, in 2008, 2007, and 2006. The effective tax rate in 2008 and 2007 was lower than the 35% U.S. federal statutory rate primarily due to earnings of our subsidiaries outside of the U.S. in jurisdictions where our effective tax rate is lower than in the U.S. The effective tax rate in 2006 was higher than the 35% U.S. federal statutory rate resulting from establishment of our European headquarters in Luxembourg, which we expected to benefit our effective tax rate over time. Associated with the establishment of our European headquarters, we transferred certain of our operating assets in 2005 and 2006 from the U.S. to international locations. These transfers resulted in taxable income and exposure to additional taxable income assertions by taxing jurisdictions. 

Our effective tax rate is subject to significant variation due to several factors, including from accurately predicting our taxable income, the taxable jurisdictions to which it relates, business acquisitions and investments, and foreign currencies. We have current tax benefits and net operating losses relating to excess stock-based compensation deductions that are being utilized to reduce our U.S. taxable income. As such, we expect a majority of our net tax provision to be non-cash. 

Non-GAAP Financial Measures 

Regulation G, Conditions for Use of Non-GAAP Financial Measures, and other SEC regulations define and prescribe the conditions for use of certain non-GAAP financial information. Our measure of “Free cash flow” meets the definition of a non-GAAP financial measure. Free