Windows 7 isn't the greatest, but it's surely not Vista (thank god it's better) >.>
XP however... is just outdated. Even with updates it's not as secure as Windows 7, also rather ugly in comparison.
If you have like 1GB RAM, yeah, go with XP.
If you have like 2-3GB+ yeah go with Windows 7.
Windows 7 is useless on 1GB RAM and runs poorly regardless of what anyone says, it doesn't compare to a 2-3GB+ system running it.
Going back to XP would be like dressing in 1999 attire and going to high school expecting everyone to like it.