>>Windows XP has built-in Remote Assistance support.
>
>Can my client see what i am doing? as she could guide me to the option where trobleshooting is needed.

Yes

>Is it always installed or does one need to install a windows component?

Yes, it's installed but has to be enabled in System Properties, Remote tab.

>Some of my clients are still running Win 98, any suggestion for that?

No, it can only be used between computer running WinXP or later.