Is there an easy way to install updates and restart automatically?

Hello there
I am reinstalling Windows rather often, but would like to do this without creating a slipstream DVD.
Windows update has this tendency to wait a couple of hours before telling you that it has found an update. Is there a way to launch the updater agent on startup and then restart the computer thus installing the updates and running the agent again?

I must say that installing updates is a pain where the sun doesn't shine since you need to watch it in order to reboot and launch the updater 6-7 times on a fresh install.
I have seen several threads about this on different forums, however none of these actually contained a download link to such a script or program.

You would do better to make a cefault install and update it - then create an image which you can restore at need.
It works out a lot less hassle, and a lot quicker, then reinstalling and updating.

That is, however, not a very good solution when you install Windows for friends or clients. They need different drivers, programmes, usernames, passwords, serials and whatsoever. It would also require me to make a new image whenever a new batch of updates are released.

Ah - I thought you were talking about a single machine, rather than may reinstalls on many machines.
Are you sure that reinstalls are the necessary actions? - most times it's simple enough to repair the problems (and a lot more satisfying <g>), unless there is a heavy malware infestation, or hardware problems.
Reinstalls on OEM hardware should always be done with the appropriate Recovery set - so realistically the best you can do is have the service Packs to hand for installation, and then use WIndows Updates.

I don't really know why they want it reinstalled. What I do know though, is that they mess it up so horribly (And some forcibly removed their recovery partition) while some also bring homebuilt computers that don't carry all these things.
The computers aren't messed up in the ways of infestation or errors, but rather just messy.
This means that they put their files in weird different places and install weird software that autoruns etc. without realizing why their computer is slow.
Surprisingly many programmes are rather difficult to uninstall because they're not packed with an uninstaller.

Finally some just want to get rid of the support software and pop ups and trials that are packaged with their computer, or maybe just want me to perform the system upgrade from Vista/XP to 7.

It's not really important to me why they want it done or wether it is necessary or a good idea at all. They ask me to do it, I tell them what happens and what can possibly go wrong (Such as incompatible software) and if they don't care I just do it.
My problem is that I'd prefer not to attend the computer more than necessary. Especially because I can chain it with other software such as automatic driver installers and Ninite and that way just run 2-3 exe files and have it reboot and continiue installation on it's own.

I hear ya!
In my case, I have the added problem that I'm on a pay-per-byte internet connection, and updating can get to be a significant expense (especially if they start failing for whatever reason).
Win7 is actually pretty good about updates - and you can usually in a clean install simply install the SPs, and then force an Update check, and install all the available Important updates at once without a problem - then repeat. I prefer to leave the .NET updates until last, and then do any driver updates necessary (since particularly Video driver packs often need .NET at one level or another). The actual hands-on time is fairly low - certainly lower than for XP - especially if you are near enough to glance at the screen every now and then.

Check this out - WuInstall. There is a free version but it won't do what you want so check out the PRO Version trail.

You might be able to work it so it reboots and continues, I know there are some reboot switches but check out the /download_cache and /install_cache switches at least you shouldn't have to download all the updates for each computer. Documentation here http://help.wuinstall.com/en/index.html. Go to Caching Options link on the left.

I can't try it out as I have no updates required but I believe the cache switches will work from a local folder if you want to copy the updates to a folder instead of connecting to a network.

EDIT: You might even want to look into Unattended installations using the WAIK or just adding the updates to the Install.wim file once a month but you can only add .cab and .msu updates. Internet Explorer 9 can be added by extracting the cab file from the exe. No extra software required to add the updates you can use DISM from the command line.

The updates will need to be added to each version of Windows you will be installing which will increase the size of the Install.wim which after a while may not fit on a single layer DVD.