Here are some essential tools
1. Anti-virus (Avast,MSE,etc)
2. DirectX
3. Microsoft dot net framework
4. Web browser (Opera,FF,chrome,etc)
5. K lite codec pack (for playback of all kinds of media files)
6. PDF reader (foxit, adobe, etc)
7. Anti-spyware (spybot S&D)
optional
1. Image viewer (irfanview, picasa)
2. Office package (open office,etc)
etc. etc.