default fonts in the internet and in other programs has changed

I just installed a whole bunch of fonts, but now some of the default fonts in the internet and in other programs has changed to one of the fonts I installed and is almost impossible to read.

I have an imac with 10.5.6

How do I change it back????

Thanks

Which version of OSX are you running please?

10.5 is different than the older versions with Font management.

im running 10.5

Use Linotype Font Explorer to deactivate everything apart from system fonts