The "Professional" version of Windows 7 is the only version which maintains compatibility with Win XP. If you purchase a computer with the "Starter" edition of Win 7, you will have to purchase the upgrade from Microsoft. Following this upgrade a free download from Microsoft may be required for XP compatibility.
Recommendation:
(1) If you have an older laptop with Win XP, use it.
(2) Purchase a computer with Win 7 Professional.
(3) If you buy a "Starter" bargain, factor in the cost of the "Professional" upgrade to your price.