Windows XP

No doubt for the last 10 years it has been the most popular operating system for PC's.
Offers good balance between performance and good visual interface.
Stability
Very reliable and stable. Some Device drivers are a bit tricky to install and might cause Blue Screen of Death errors.
Security
Like any other Windows version highly open on virus infection usually when using the integrated internet explorer.
Speed
Very light and doesn't need a lot of computer power to run it fast. It will work fine on computers with only 512mb or 1gb of RAM.
Problems
- Naturally doesn't support wide range of plug and play devices
- Some drivers a bit tricky to install
- windows installation is not naturally supported for sata Hard drives and requires some technical skills
Bottom Line
Ideal for people that are not tempted by the visual effects and interface of Vista and Windows 7 but want to get the best speed from their machine. Only choice for computers older than 3-4 years.
Always use in conjunction with a good anti virus like
MalwareBytes
Windows 7

Release only 1 year after Vista this is only a fixed windows vista version with better speed and stability, so for most people XP to Windows 7 is the most reasonable upgrade.
Stability
Doesn't suffer from the Vista Black Screen of death KSOD, but strange enough programs fully compatible with Windows Vista will not work very stable on Windows 7. Stability has been definitely improved, so now it is up to software developers to update their software with a Windows 7 Stable versions.
Security
No improvement Here at all over Vista , but a lot less Dialogs and warnings about the Users Security Management.
Speed
This is one of the main vast improvement over Vista. A computer with only 1-2Gb will work with usable speed and will load in few minutes. They have achieved this by removing some non essential start up processes from loading on boot up.
Problems
- Missing DVD due to a corrupted drivers is the only problem that has been inherited from windows Vista. All other issues has been cleaned out.
Bottom Line
Same fast and improved search functionality like vista , but still confusing for finding files of a certain size and type. Doesn't require very expensive computer to run normally and you benefit from the nice Aero windows , menus and transparent effects. Requires quite a bit of storage space, but these days most modern computer have allot more than need for a normal home user.