Comodo's Time Machine is a software application that runs on your Windows machine and periodically (either manually or automatically) takes snapshots of your system. You are then able to roll back to any of these snapshots in the future. Indeed you can jump backwards and forwards in the tree and new branches appear as you make changes to the system.
The idea behind it is that if you suffer any problems with corrupted software, malware, etc., then you can roll back to a known good state and start again. You can lock snapshots so that they don't get deleted and then clear out the ones that you don't want to keep any more. This is quite important, especially if you take automatic snapshots. You have to remember that every change made to the computer (i.e. every time you run it or change a file) the changes are stored. When a new snapshot is created, if you change a file you will have a new version on your system as well as the old one. Due to this, it requires a fair amount of space on your system. However, the upsides are fairly obvious.
I have been using it quite a lot recently on test boxes while performing testing of security software against various malware and other attacks. It enabled me to perform a test, roll back to the pre-test state and perform it again or try another attack from a fresh system. It greatly reduced the testing time for certain attacks as I wasn't having to deal with an imaging server, etc. For the normal user, however, this does mean that if you get infected with malware or something else goes wrong with your system, you can very quickly and easily roll back to a previous state and carry on working.
There are a few issues to keep in mind though. Firstly, as I've already mentioned, the space required can be quite large if you keep taking snapshots and don't clear previous ones off the system. Secondly, if you roll back your system, you won't have access to any new files or software that you have put on the system - you will need to roll forward again to get at these. Finally, I did have one or two occasions where the restore failed. When I say the restore failed, I mean one snapshot failed so that I couldn't boot into it. At the boot stage I had to select another snapshot to boot from. I could always find a snapshot the did work, but it is slightly worrying that there were occasions when the one I wanted wouldn't boot. This could be due to the fact that I was installing various service packs, updates and malware onto the system and switching between them many times, but it is still worth noting that you will require a full system backup and you must backup all your data regularly.
Of course there are other products out there that do the same thing and some reviewers say that they are better (e.g. Acronis). However, I found Comodo's Time Machine very easy to use and it is free. I'm not necessarily endorsing Comodo's product; I'm saying that this type of software is worth a look for keeping your systems running.