First Microsoft used warnings and migration tools to help make Windows XP users upgrade. Now, it's trying a different tack: bribery. In an effort to get folks to stop using the 12-year-old OS, Microsoft is offering XP hold-outs a $100 discount off new Windows 8 PCs. That sounds like a sweet deal -- and it might well be -- but there are several catches you should be aware of. First off, the promotion only applies to a select few models, and you'll need to visit a Microsoft Store to even see the full list.Secondly, the offer only applies to PCs priced at $599 and up, so if you thought you could use this to buy an extra-cheap tablet like the ASUS Transformer Book T100, then you're sorely mistaken. (The Surface 2 isn't included at all, which means even the $679 LTE model is off limits.) Finally, the deal is only valid through June 15th, so you'll need to nab the discount soonish if you intend to use it. Regardless, with extended support for XP ending on April 8th, two weeks from now, we suggest you upgrade ASAP if you haven't already.