I am just wondering can wubi 11.04 make windows unbootable in anyway and what harm can it do to my PC. Also, if I force shut down in wubi ubuntu can it damage windows?
WUBI is pretty safe: it creates one file on the Windows disk, and new boot menu entries, but doesn't change partitions. You should always be able to boot back to Windows, and an abrupt shut down might corrupt the Linux filesystem but nothing outside of that.
Nothing in life is entirely without risk. You should, as always, have backups.
The main kind of risk that Wubi mitigates is needing to repartition, or reinstall, or to install a new boot sector. Those aren't very risky, but they can be hard to undo if you instruct the installer to do the wrong thing (eg to delete Windows) or if you change your mind.
Wubi is the safest way to have a second OS without having to make irreversable modifications. I have yet to see a noticable difference between a Wubi install and a native partition. The downside is that it requires Windows for Ubuntu to operate. A native partition is the better option, because if Windows crashes, you can boot-up to a working operating system.
Ubuntu is very powerful and it makes it easier to take control. You can just as easily be productive as destructive. Windows (or a Mac) are designed with many more precautions that make it harder to accomplish basic tasks, but harder to hang yourself. With Ubuntu, you kind of have to know what you're doing to a much greater degree than if you're using a commercial OS.
I probably did damage to the file system of my "safe" computer, the one I use for backups because it has a RAID 1 disk configuration. I didn't know that RAID is not supported by Ubuntu. Maybe I missed it somewhere, but I tried reading about the install for hours before I actually did it, and I didn't find out about this unsupported configuration until after I had done the damage and was searching through the help.
This OS definitely gives you a lot of power, but it could also give you just enough rope to hang yourself!