PC Advice

We had 2 hard drive crashes within days of each other (2 of our 3 home machines), and just received the recovery data from the latest on DVDs. I have several questions, so could use some advice from a PC guru (or two):
1. Can I load the recovered data (from XP) onto our Vista laptop?
2. What should we be doing for maintenance to prevent these problems in the future?
3. The latest crash is from an old PC that's been limping along for many years, replacing parts as they break. By the time we buy a new internal hard drive & get it attached, the next part may fail. At what point does it make sense to buy a new PC?
4. What do you recommend for firewall, security, etc? At what intervals (or automatic)?