A lot of factors affect the boot time. Firstly, how many programs you might have in the computer. It clogs up the registry and windows needs to traverse the entire registry while booting.
At the time of the boot, there may be too many programs starting up. This causes Windows to slow down even further since certain files are requested by these programs, hence using up the memory windows would otherwise have used.
Free space on the hard disk is also essential because windows makes page files on the disk for its cached files, and if there is little space left, the caching is improper, leading to slow boot times.
Also, the page file should not be set too high because generally (unless you have a SSD), hard drives are much slower than the RAM, hence, more Windows needs to use the disk, slower it performs.
By boot, you mean the windows logo to the logon screen or the logon screen to the fully functional desktop?
If the former, the problem is likely to be in the registry, and there is not much you can do about it, because all the programs you might have installed require these entries.
If the latter, all the other things I've highlighted are likely to affect boot times.
Hope this helps.