My father's Safari doesn't seem to be quite right.
See the screengrabs below -
http://img.photobucket.com/albums/v634/ ... re1-18.png
http://img.photobucket.com/albums/v634/ ... ure2-9.png
http://img.photobucket.com/albums/v634/ ... ure3-4.png

I'm posting this from Firefox on his machine, and all is fine in Firefox.
I've deleted the Safari pref files and restarted/relaunched, emptied the cache, checked the accessibility options aren't doing something weird.

Any ideas?

I have been have similar problems. Though Safari hasn't changed recently. I thought it was down to a DNS problem. Some times a fresh reload resolved it.

Have you checked the Extensions (assuming it's Safari 5)? Try turning them off altogether in Safari > Preferences > Extensions, just in case there's one that's blocking the content.

Have you tried rebooting the router?

I have no extensions installed at all. I have refreshed the router but this does not alter the behaviour. It is gets back to normal without any reset of modems or routers.

I've restarted the machine (of course), to no avail. I've downloaded a new copy of Safari from Apple, and installed that. To no avail.
The router... I'll give it a go, it's an ADSL modem/router that's feeding an Airport Extreme for the wifi. He's connected via Ethernet from the Airport Extreme though.

I'm not having any issues on my side of things, nor is my sister on her Vista laptop (both wifi connected).

We do run AdBlock, I'll give it a go. I did see that there was an update to it waiting, but performing the update hasn't cured the issue.

Well, disabling AdBlock seems to have cured it. I'll have a dig at it tomorrow.

Check to see if there isn’t some obscure custom CSS file being used. It looks like something is overruling CSS from the sites.

Go to Safari>Preferences as see whether something is selected in the Style Sheet drop down.

Yeah, I did have a poke at that, but there's nothing showing.
I'm wondering if the block list that AdBlock was subscribed to went a bit mental?

in safari on my system i use the elements.css
preferences > advanced > style sheet > drop down menu > elements.css

also to disable DNS prefetching open a terminal and write (copy & paste)
defaults write com.apple.safari WebKitDNSPrefetchingEnabled -boolean false
press enter

to clear the DNS cache write (copy & paste)
dscacheutil -flushcache
press enter

restart or reopen safari ...

as an add on
also check the top sites part of safari as it can store hundreds of sites
also the top sites cache as it can run into GiBs of storage ...

What those screen shots tell me is that CSS is not being loaded. Next time you are there, open one of the pages that break and then go to Window>Activity and see if any files are not loading. The URL they come from may give a clue.

I use OpenDNS to block ad servers, and About.com’s CSS never loaded until I whitelisted it.