I boot from a 4 year old 160GB Hitachi drive. I have three OSes installed and none of them boots slowly. Windows boots within a minute maximum, often less than 45 seconds. For faster bootups, better remove startup entries and clean your system using a utility like CCleaner. I have only on startup entry, i.e., Antivirus.
For a $100 budget, I would rather buy a backup drive as it is much more useful. Even if I do buy an SSD, what will it do? Boot my PC in less than 20 seconds? I can wait 20 seconds more and boot with a normal drive. The fact is that SSD's are still not mainstream and make a very bad value for money.
Thinking of $100, I think I would buy a Sansa Clip+ PMP and Soundmagic PL-21 IEM's. Would be awesome value for money.