I can guarantee out of my own experience that a 2.8GHz Pentium 4 will not make a computer noticeably slow. In gaming, yes. In basic tasks, no. I have used a Pentium 4 520 before and know for a fact that as long as your computer is clean, it should run pretty nicely. One other thing I recommend is hitting <Windows Key>+R and typing "Msconfig." Then go to the startup tab and search all your different startup programs on google. If any of them are unneeded, then uncheck them. Sometimes it will display some error about administrative privileges, but in my experience it makes no difference. Also, one thing that used to slow down an old Pentium 4 Dell I had was its RAM. Do you only have 256mb?
Also, @ one of the previous posters, a 2.26GHz Pentium M is faster than a 2.8GHz Pentium 4. The Pentium 4 is a Netburst processor, which has a long instruction pipeline, therefore reducing how much it can process per GHz. The Pentium M is a P6 processor, which has a much smaller pipeline, therefore allowing it to be much faster per GHz.