Hi,
I was browsing a site that tried to install an embedded video player when avast popped up telling me a trojan was detected, so I quickly hit the ‘abort connection’ button!
Looking at the web shield page I now have infected:1 displayed, but nothing in the virus chest.
Do I need to do anything else or is everything handled already?
Thanks in advance,
LW.
Siince Webshield stopped the infection before it reached your hard drive, there isn’t anthing to quarintine. You did the right thing and you’re ok. 
The web shield page is just reporting the last file scanned, the last infected file found, etc. they are just stats. What you should notice is that the location of the infection is an internet URL, meaning as oldman said, the web shield intercepted a file on the internet and stopped it from being downloaded to your system.
You will have received an audible and visual alert and the only option on that alert is abort connection (see image), e.g. drop that download (not your whole internet connection), so the file shouldn’t be on your system.
Great stuff, thanks for your help 
Your welcome.
It’s handled already… but it won’t harm if you run a full avast scanning (or boot time scanning) just for safety 8)