By all accounts posted here, it should not be necessary to reformat.

There are other things that can and should be done first.

Contact your internet provider, and explain the problem. Make an appointment with a tech service at a local shop, or have the Geek Squad make a visit, whatever.

If you start doing things you personally don’t understand, you could be digging your self even deeper into the hole on this.

XP wise, this is a "no brainer". Shogun works on XP fine. There is something else wrong. It could be as simple as something wrong with your copy of STW, or it might be with your internet provider.

Have you tried other Total War programs, and verified that they do or don’t work with your computer and connection?

Have you picked up another copy of Warlord (or Shogun) yet, to give that a try?

Please do the simplest and least expensive things first