The bottom line is that Microsoft Windows Vista quickly developed a ridiculously bad reputation - Ford Pinto bad. Droves of early adopters retreated back to the safe confines of old but reliable Windows XP, and laptop manufacturers even offered "downgrade options." Microsoft doubled its efforts to make Windows 7 its salvation, and Win 7 has largely succeeded in that regard. While most of the attention has been directed at Win 7's improved user interface, there are still several questions about its performance on older computer systems.
The CPU landscape has changed dramatically from 2006 to 2010, and while it wasn't uncommon to be using a single-core CPU four years ago, it's next to impossible to find a processor with less than two cores nowadays. Quad-core CPUs and HyperThreading have entered into the lexicon and a lot of software is now written to take full advantage of current multi-threaded chips.
Any major computer upgrade generally requires a full software reinstall too. No so if you heed the advice in the guide to Upgrading a Motherboard without Reinstalling. PCSTATS Tips