Just a stab in the dark here, but when you look at the display properties for your machines, what do you see when you right-click your desktop and choose Personalize, then click the Display link in the lower left corner where it says See Also?
Does the dialog report anything beyond 100%? If so, try setting to 100% and see if that resurrects the visibility of the guide.
One other thing that may help. If you right-click on the guide, the context menu offers an option to change from Pixels to Percentage. And when you do that, the guide changes color. So it's possible that if you have done this, that particular color isn't standing out too much because more contrast is needed.
Thanks for the swift response, i've checked both your options and those aren't the problem, i found my guides when i zoomed to 300%, so i started playing around and found that the rulers apparently aren't long enough because they stop at about -4000px on the y axis at 100% zoom. No idea how that happened and why it happens on both machines but they can be used even though they're annoying to use right now