dual boot is the best if your not completely sure...
try the live cd first, if you like it then install windows or clean up a partition and install ubuntu use it, ask questions, try some apps, etc. you may find that there is almost nothing that can be done is windows that not be done in linux, however if you like to do your taxes on a pc I recommend keeping windows or using a virtual machine, the biggest loss I suffer from a linux only system is not being able to play a handful of games I bought for a windows pc, but since I normally dual boot(install windows first then linux it's much easier that way) I can still reboot and play them in xp anyway so no loss other than about 60gb of HDD space. I'm sure the other posts say similar things, I decided to be an asshat today and skip the others, I know I'm probably going to regret it later though. also, good choice going with ubuntu, its very nice, especially if your just switching from windows, good luck.