You can dual boot by using a bootloader. Either LILO or GRUB.There are other ways but one of those will be the easiest.
Yes you can dual boot with linux and windowsXP.
The only extra time it will 'lag your computer down' is the time it takes you to choose which OS you want to boot to.

What Linux OS are you going to be using? If you havent decided yet and it's your first time, I would suggest using Redhat, SuSE or maybe Mandrake, until you get familiar with the environment.
Are you starting with a clean drive or do you already have a hdd with windows on it that you want to repartition?
if it's clean, I would suggest partioning it in half, maybe even give windows some more room since its bulky(I suggest using FAT32 for a file system, since then you can read and write data to your windows partition from linux, if you use ntfs you can only read the data). Instal windows
first, do what you need to do to get it up and running. THen install linux, during the installation it will see the windows installation and most Linux OS's will automatically set it bootable and configure a bootloader selection screen for you. Choose the unused partition to instal linux on. If you use a linux distro like slackware and use cfdisk to partition your drive just leave the windows partition intact and set it bootable, later in the installation it will allow you to choose and configure your bootloader.

If you already have an existing windows partition that takes up the entire drive, you have two choices:
1. partition magic:$$$
2. do it yourself:free

I never used partition magic so someone else can help you with that if thats the way you decide to go.
If you want to do it yourself, Ill warn you now. You can potentially completley fuck up your existing installtion if you dont do this correctly. First remove all uneccessary files. then defrag. make sure all data is moved to the beginning sectors of the drive. write down how much space is taken and how much is left. give yourself extra room for swap(windows calls it virtual memeory) and space for extra storage. then you need to resize the partition to the size you determined suitable. this is only available under the expert install or mandrake and maybe redhat and suse(can anyone verify this? resizing partitions that is). then just use the spave left to create a new partition to install you linux distro in. =D