I've been using Photoshop CS5 for a while now and one thing that is really annoying is the width of the guides. In CS3 their width was only one pixel but now it seems that they are a little bit wider. How can I change their width?
Edit > Preferences > Guides, Grid, & Slices... you can change the Style of the line. Dashed Lines will have a lighter appearance. The color can also affect the apparent width of the line, recalling both human perception and the separate RGB elements of displays.
If these settings do not improve the appearance of the lines, it is possible that the issue lies with OpenGL rendering. Open
Edit > Preferences > Performance... and if Enable OpenGL Drawing is checked, open Advanced Settings and toggle Anti-alias Guides and Paths. You can also try deselecting Enable OpenGL Drawing but you will lose a number of features.