The big advantage of using a Virtual system over a dual boot system is that you can have any number of Operating Systems running at the same time. I have one machine where some of the setup is easier in XP and some in Linux, with a multiboot system I would be forever re-booting to get to the system I need.
The VirtualBox instructions are confusing, below is a step by step, hope it helps.
Start Virtualbox
Click on New and the New Virtual Machine Wizard opens.
Click on Next.
Give your new virtual machine a name i.e. WinXP and select OS Type as Windows XP.
Depending on how much memory your system has acccept the 192 suggestion or increase it to whatever you want. I use 256 on on my system. Click on Next.
You now need to create a virtual hard drive so select New.
The Create New Virtual Disk Wizard opens. Click on Next.
Accept Dynamicaly expanding image, click on Next.
Give the virtual hard disk a name i.e. WinXP, accept the 10.00GB size. Click on Next
Click on Finish to go back to the vitual machine setup.
Click on Next and then on Finish.
You will now have a Virtual Machine called WinXP in the list of machines. Click on it.
Click on CD/DVD-ROM and tick the Mount CD/DVD Drive.
Select Audio from the list and tick the Enable Audio, select Windows DirectSound from the drop down box.
Click on USB and Enable USB Controller, enable the USBEHI Controller.
Click on Shared Folders and then click on the thing that looks like a camera on the right side, it actually a + sign on top of a folder. Choose which bit of your existing system you want to share data with, click on OK, click on OK.
You are now ready to install XP. Put your instalation disk into the CD/DVDdrive and Click on Start to start the virtual machine. Vista will ask about autoplay the CD, just dismiss the dialog.
XP installation will start.
Once XP is installed and until you install the Additions, press the right hand CTRL key to free the mouse from XP control back to Vista control.
Formatting may takes a while even when it gets to 100%
Wait for XP to install.
Answer XP questions and setup XP as normal.
Wait for XP updates to download then install them
Wait for XP updates to download then install them
Mount the guest additions ISO image by going to Devices, Mount CD/DVD-ROM, CD/DVD-ROM Image and select VBoxGuestAdditions.iso from the list.
If a Window opens then double click VirtualGuestAditions, otherwise go Start, My Computer and double click VBOXADDITIONS_x_x_x_xxxxx. Continue anyway when XP complains about a license.
With Guest Additions installed the mouse will auto-capture and release and you can drag the sides of the VirtualBox window to change the size.
Start, Control Panel, Classic View, Network connections - Set up Home or Office network, Turn On File and Print sharing.
Start, My Computer, My Network Places, Add a network place, Choose another network location, browse and select Entire Network, VirtualBox Shared Folders and the the folder you choose to share earlier and then OK, then Next. Give this a few minutes to work though sometimes it stalls, if it stalls then just re-run.
Start, My Computer, Tools, Map Network Drive Browse to Entire Network, VirtualBox Shared Folders and select the folder you chose to share, sometime you have to go back and select again to get OK to appear. Finnish.
Create shortcut to Device Manager C:\Windows\System32\devmgmt.msc on the desktop to give easy access to the Device Manager.
Install USB to Serial drivers on XP system, then Devices, USB Devices and choose the USB to Serial device from the list. Test the USB to Serial Converter if it doesn't work try a different make/type.