I've used Virtual Box now for a while and have found it very good,
it's also free so nothing to lose by trying it.

Note that there are drawbacks to virtual OS's as some programs need to
access the hardware directly and in those cases you do need to run
Windows natively on the hardware, whether dual boot or not.

I use Oracle VM Virtualbox from
http://dlc.sun.com/virtualbox/vboxdownload.html

VirtualBox is much easier to set up than VMWare. There are two versions of
VirtualBox, OSE which is the repositories (don't use it, unless you are hung
up about open versus proprietary software) and PUEL (available from the
VirtualBox.org site).

PUEL gives you access to usb devices and OSE does not.

Three votes for Virtual Box is good but I've been using it so long I'd
forgotten about the USB thing - thanks for picking that up :-)

PUEL gives you access to usb devices and OSE does not.

I forgot to say that both are free (of cost) to use. Only OSE is free (as in
freedom). PUEL stands for personal use and evaluation license. OSE is open
source edition.

VMWare Server and player are free (no cost) to use, but neither is free
(open source). Their other products are pricey, though.

Another vote for Virtualbox. Don't get the one from the repositories, since it
has a couple of important features disabled.