Quoting from MacWindows.com

Ed Welsh bought a MacBook Pro to use as a Windows machine in Boot Camp. After discovering Mac OS X, he moved his Boot Camp-installed copy of Windows into a virtual machine in VMware Fusion. He was then able to delete the Boot Camp drive partition: 

I have been using VMware Fusion for about a month and really like it. (I have never used Parallels.) I first started using Apple computers about 6 months after the Intel MacBook Pro came out. I've used Windows and Linux since the beginning of time.

My MacBook Pro started as a Boot Camp system that spent all its time in Windows XP Pro. I had 75 GB dedicated to the Boot Camp partition and 35 GB holding OS X. I didn't use the OS X side at all, until I needed some Linux utilities for a job and booted to OS X to see if it could use them. Everything worked flawlessly and I have been spending more time in OS X ever since.

Now I wanted to get to both OSes as needed without booting back and forth. During my research I found the free VMWare Fusion Beta that supported Boot Camp partitions. It worked flawlessly.

I recently decided to move my Boot Camp Windows install to a virtual machine and do away with Boot Camp. The reason was backup: I had to use a NTFS formatted backup disk for Windows and HFS+ for OS X. With my main Windows installation running as a VM on top of OS X, I can backup everything to HFS+.

Here is the process I used:

1. Used Windows Backup to backup my BC partition to external drive.
2. Created a clean 70GB VM with Windows XP Pro (600MB zipped)
3. Zipped that up and put it on an external drive.
4. Used Boot Camp Assistant to restore the MacBook Pro to one partition (111GB).
5. Extracted the fresh Windows virtual machine from external storage to MBP
6. Used VMWare Shared Folders to share the external storage with new virtual machine. Started the new virtual machine and used Windows Backup to restore from external storage.

This method trashes the BOOT.INI used by Windows so the first boot after restoration should be done with a bootable Windows XP Pro CD (or ISO) available. I just boot the VM to the Windows XP Pro CD crank up the Recovery Console and use "bootcfg /rebuild" to have it recreate the BOOT.INI file. It will boot perfectly after that.

Now my Boot Camp partition is a regular virtual machine sitting on my OS X system in neat little 2GB chunks, ready for backup.

The MacBook Pro and Fusion never skip a beat (2GB RAM). Sometimes I get some HD contention, but only when I run two hard-drive-intensive operations on separate virtual machines at once.

If you've tried this method, " + contact + "") //--> please let us know how it worked for you.

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