seclogon is for any kind of secondary log ons that you have on your computer. If you disable it then obviously those will fail to load and you can't use them. If there really isn't anything depending on that service then you should be okay to disable it.
If you happen to disable something you need, you can just turn off your computer and turn it back on. Before actually disabling the process, just turn it off to make sure it doesn't mess up anything on the computer. If it doesn't mess up anything that you normally use, then you should be fine. When you restart the system it should turn back on. As long as you don't disable it from starting when Windows is started.
I am going to say not to disable lanmanserver. This is something that turns on during start up and looks at the configuration of your Hardware and makes changes to the Service if necessary.
As for iphlpsvc, it offers IPv6 connectivity on a IPv4 network. I am pretty sure this has something to do with websites..? Not exactly sure on this one.
To disable gpsvc, your going to need to delete something out of your registry and then re boot the computer. Here is what you will do; Go to Run, Type RegEdit, press Enter. When that opens find this registry key "HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\gpsvc\". Delete the Services key and then reboot the computer. This will reset GPSVC and enable you to disable it after reboot. Remember to do this at your own risk and be sure to make a system checkpoint before doing this. Just to be on the safe side
CertPropSvc is a service that allows Smart Cards to supply trustworthy root certificates which, among other uses, can be used as a method of logon in Vista. If you don’t use Smart Cards, you don’t need this service, though because it’s set to manual by default, you won’t gain anything by disabling it. So if you don't use them, you don't need it.
I'm not a pro on computers or anything, but I'm at least trying to throw out something so you know what you are working with.