i have the windows xp instalation cd but i try many ways to format c but i can´t i really need help my computer is full with trojans

If you are reinstalling XP just boot the CD & follow the prompts. XP will do its' own formatting. Here is a good guide to either a Clean or Repair install depending on your choice:

http://michaelstevenstech.com/index.html

Let us know if you have further questions.

Just to add to jephree's excellent advise, here is another link.

Performing a clean install of Windows XP

Just to Add:
MS do a set of 6 floppy's you can use to set up your PC for XP install.
You need the ones for your CD version of XP, e.g. No SP, SP1 or SP2

http://support.microsoft.com/default...;en-us;Q310994

As a little extra advice get these now if you need them and put onto a CDR for installing before you go on the net.

MS Anti Spyware Beta (Set to Auto Protect on install, stop homepage hijacks)
Spybot Search and Destroy
Zonealarm (Firewall)
AVG (Anti virus)

All are free from http://www.filehippo.com/ in the PINK middle section