Why does the blue light next to the power button blink on a Windows PC?
As usual, Jan offers sterling advice - check the manual! Although on most computers a little blinking blue light indicates hard disc activity, this in not universal, and even when it does, the details can vary. If you don't have a printed manual then it will be available electronically either on the computer itself or on the internet (simply google the computer's model name followed by the word manual)
Assuming its your hard drive activity light then its blinking when your hard drive is active. As others have said more information is needed to correctly answer your question.
We would need the make and model of the computer to be able to find out what the light is for. Typically a hard drive usage light would be orange or amber in color, but it may be blue on some computers. These lights show if the hard drive is being written to our read from along with his hard it is working. Other lights could be Wi-Fi/networking connection, Bluetooth, or other connectivity device. These may flash to show how much traffic or usage is being used going in and going out of the computer. These are just examples of what the light could be for. More information is needed to determine what it more likely is.
I suggest you check your computers or computer cases manual to see what this light is supposed to signify.
On a desktop system it's normally the hard drive activity LED. It should blink whenever there is activity on the hard drive. The reason I said should is because there are situations where the light might not be blinking at all or even be constantly be on.
For example depending on the mainboard layout the LED may be shared between hard drive and optical drive activity. While not relevant in this case the LED may also be limited to certain ports e.g. only SATA0&1 or only the IDE ports but not SATA ports on the motherboard.
Certain tools or even the operating system may regularly poll information from the disk causing the light to blink. Windows Vista prior to SP1 used to poll information every XX seconds for security reasons. Windows Superfetch will obviously cause activity when it is working on prefetching your frequently used programs. Volume Shadow Copy (the process for backups and previous file versions) will cause hard drive activity when it get's active, System Restore, scheduled disk defragmentation, indexing, etc.
was it always blinking?