Physical to Virtual on same hardware?

It should be possible, but it's difficult without knowing for sure that your hardware is is supported. I'm not going to say that it will work only to have you hit a snag and have it not work, then complain...in some circumstances it might.

The smarter thing to do is get a newer server dedicated to virtualization, then migrate your existing server over and decommission it.

Especially since your server is your only one. SBS will potentially die in the process, and leave your business high and dry.

Here's the options as I see it. One, make sure you have known-good backups, and verify that your p2v migration will support sparse disks, since if it will create the same size volumes you're going to have a size crunch. Then you can allocate a few days to trying it, and if it fails horribly, restore the computer back to the previous state.

Two, which I'd say would be safer, get a server with tons of storage dedicated to being a VM server, migrate your server to that, then decommission it once it's working well. Then turn your old server into a backup domain controller. Even for small business, it doesn't hurt to have 2 DC's.

Three, don't do this, unless you really have a compelling reason to move to a virtual system structure. What are you doing that you are running with SBS, but want to virtualize on that server? Especially if you don't have extra hardware for availability/redundancy? If you do idea two, you could migrate your server, make sure it works, and then turn the old server into another virtual server and install a backup DC and...something else you're trying to virtualize.

Depends on what exactly you're trying to achieve as an end goal (for what reason...?)

I did this at a church that had zero budget for hardware, but wanted to migrate to SBS2011 on the same server. It was risky, but it worked.

1. I installed the Hyper-V role on the SBS2008 Box
2. I used disk2vhd to make a copy of all the disks on the server, onto the same server. It uses snapshots, so you can have the source and destination be the same disk
3. I mounted the virtual SBS2008 on the same machine as the real SBS2008. The important thing I did here was I did not give it a network adapter. The only reason I did this was to ensure that my copy did in fact work.
4. I did a Hyper-V "Export" onto an external drive, and completely wiped the SBS2008 server
5. I did a Hyper-V "Import" back to the main machine after I had a fresh 2008 R2 installation with Hyper-V added to it
6. I booted the new Hyper-V machine and have been happy ever since. I should have virtualised it in the first place.

I was then able to install SBS2011 in a second VM on the same hardware, migrate everything over, and then just turned off and disabled the original SBS2008 VM. It took a fair bit of time, but it meant that we didn't have to spend a single dollar on hardware.

Important things to note:

1. This only works if you have a single domain controller (the SBS machine). Because your AD will effectively go backwards during this (from the snapshot), if you have more than one AD they will complain about "Invalid restoration" (same as if you rolled back to a previous snapshot of a VM). Then the "old" DC will then just sit there being useless forever. I did ours on a weekend when almost all the computers in the office were shut down. We only had an issue with one network printer that (still) refuses to re-join the domain.
2. You might have licensing issues. We did this on an educational license, which is not OEM, and thus we were allowed to virtualise. My work also donated the 2008 R2 license that we used as a hypervisor, but the vanilla Hyper-V hypervisor is free if you don't want a fully functioning OS underneath it.
3. The first import of the VM is important, because you don't want to nuke your SBS2008 host without making 100% sure that your copy is in a good enough state to work.

KEY question #1: Did you buy SBS preinstalled on the server? If so, you CANNOT virtualize it. OEM copies cannot be virtualized. If you bought a volume license or retail copy, you CAN do this - those can be virtualized.

Assuming you don't have an OEM copy then I ask KEY question #2: Are you suggesting you have NO other computer with at least the capability to have 8 GB of RAM and a virtualization capable processor? Almost EVERY new computer sold in the last couple years and MANY in the last 5 support this configuration. At worst, you could build one for $400.

You can migrate using third party software like ShadowProtect or DoubleTake among others... these are not inexpensive solutions.

If your TIME is not valueable, then you can do this the by doing a swing migration to a temporary server. It's like an upgrade only you're staying on the same version. You install another system with Hyper-V, migrate to that server, then reload the physical server with Hyper-V and export the VM from the temporary server and import into the newly installed physical server running Hyper-V.

I did basically this exact same thing for a client in October (only difference, is we upgraded them to SBS 2011 Standard from SBS 2008 Standard. But logically, it wouldn't have been different to migrate to SBS 2008 (the same version).