a couple of questions... first, can i run ubuntu and windows xp on the same system? second, does ubuntu let you run a tv-tuner?

1) Yes you can. Its called dual-booting. Its a great way to get started with ubuntu or windows as you can take a 'dip' in the shallow end of the pool and not get too wet. Personally, I found it easier to simply jump in the deep end and be rid of Windows entirely. You can re-install Windows in a virtual machine and run it at full screen if you get withdrawal symptoms, but I know alot of people dual boot to run games and such, as virtual machines dont handle 3D programs. Heres a couple guides to get you started,

https://help.ubuntu.com/community/WindowsDualBoot
(its long, but DO READ every little bit to avoid any troubles.)

http://video.google.com/videoplay?docid=-6104490811311898236
(Not very official, but still, handy to see the steps, rather than read them)

2) This is a tough one. You need to specify WHAT tv-card you have. I have an el-cheapo TV card that works with a little bit of tweaking, but alot of cards work out of the box. TVTime is the favorite TV viewer by most folks, as its simple yet full featured. If I were you, I'd find the name of the tv card and google it in this manner "bttv tvcardname", as bttv is the driver backend for alot of TV cards. Failing that, just post another support question asking if your Tv cards name is supported in linux.