If you open the email in a web client (ie, online at gmail.com or mail.yahoo.com, etc), you're generally very unlikely to experience any problems. If this email contained a script virus (very rare nowadays) it would generally require being opened in an email client locally installed on the computer in order to gain sufficient access to actually infect your computer.
Viruses, because of the popularity of web clients for email, have pretty much stopped sending themselves as emails in the last few years.
Spam is still a problem, and many viruses create spambots and enslave their infected computers as spam relays. But you're not going to catch a virus from the average spam message.
If you are using a local email client, don't open suspicious emails unless you've got the email client running inside a virtualized system that you can scrub easily with a reset.
It could also be:
3. HTML page with JavaScript code attempting exploit a vulnerability in your browser.
4. HTML page with an embedded Java applet attempting to exploit a vulnerability in the JVM
5. HTML page with an embedded Flash file attempting to exploit a vulnerability in Flash Player
6. The email itself, before you open the attachment could try to exploit a vulnerability in your email client
There might be other possibilities.
For this purpose, I have the following setup:
Virtual Machine using VirtualBox. No network access.
I have a snapshot saved for the VM after a fresh OS install.
I also take two snapshots with What Changed? and TrackWinstall.
I copy files only in the direction Host -> VM, using a free ISO creator.
I create the .iso
I usually run the malware and study memory usage, CPU load, listening ports, networking attempts.
I check the changes to the OS using What Changed? and TrackWinstall.
Finally I restore to the fresh snapshot.
The reason I have the whole setup is because I like to run the malware and see what it's trying to do.
Update:
I was talking to a colleague who performs malware analysis as a hobby and he told me about his setup, it might be different that what you might want for an occasional .html
Old PC with a fresh OS install.
After installing the needed tools he takes a full-disk image using Clonezilla Live.
What Changed for snapshots comparisons.
The PC is connected to the Internet through a separate network.
Whenever he finishes working on a sample, he reboots with Clonezilla and restores the full-disk image.
