Well to be honest Vista requires a more powerful machine than the previous version of Windows did (called Windows XP).
Some companies are selling computers that are just not powerful enough to run Windows Vista.
Assuming it is your LAPTOP that is slow (rather than, as Ethel says, your internet connection) then this can be caused by 2 main things.
1) Your CPU (processor). Some CPUs are faster than others and if yours was a cheap PC it may have a slow processor.
If it is a slow CPU then I am afraid you are stuck with it as it is difficult to add a faster CPU to a laptop.
2) Your memory (RAM). If it is a new PC it should have about 1Gb (Gigabyte) or more of memory. It is fairly easy to add more memory to seepd up a PC.
If you can let us know what CPU you have and how much memory will can let you know if it is powerful enough.
The CPU will be called something like Intel Dual Core 1.6Ghz or maybe AMD Dual Core 1.8Ghz (something like that).
The memory will be maybe 512Mb, or 1Gb or 2Gb.
Let us know.