no application to open some files

I have a hp/win xp/cable. Around the first of the year I started having
trouble with it running slow. Then it started freezing up. I tried to
restore at that time, but it still won't allow me to restore to any date.
Now it will not open up some pdf's and other files. Finally 3 nights ago it
froze up and I couldn't even turn it off with a hard boot. I ended up
unplugging the whole computer. Dangerous I know, but I had no choice. I
would really be grateful to anyone who culd help me out. It is imperative
that I get it running right, because I depend on it to run my business.
Thank's again for any help.

Roxie