update
Hi
OK
this is the worst nightmare for any developer! We have all been there.
YOu need to find out exactly what browser and what version your client is using - some old browsers do not render CSS the same
Next find out what resolution they are viewing at and at what colour depth.
I design for 800*600 px in the UK because thats the most popular resolution but I am moving towards 1024* 768
Then test using their settings. Make sure they are not using AOL because AOL can render in weird ands wonderful ways.
If that doesn't profer a solution then do one oer all of the of the following:
1. Take a laptop on site ans show them it's working OK
2. Tell them to look at it from home
3. Tell to look at it from a PC shop
4. Ask themto get their friends to check it.
5. As a last resort post a link on a forum and ask peopel to check it for you and post their specification.
If 20 people respond you at least have 20 posts to forward to the client.
It is a pain I normaly find it's because thie resolution is to low. Remember all websites look different at various resolutions.
HTH
Ian