"Start>Accessories>System Tools>Disk Defragmenter" is the place to look. Run the analzer to see what the percentage of fragmentation is. If you end up seeing some 10% or higher that could be the problem.
Your first step should be to make sure you don't have any malware running in the background. Go through osiris' guide to make sure of it.
Bit Defender is not the best AV, but it is not the worst. I will sometimes use BD's online scan as a backup to verify if a computer is clean. I, however, am not an advocate of free AVs, as you get what you pay for. I would recommend trying Eset's Nod32 (or internet suite), TrendMicro's PCCillian, or Kapersky.
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It certainly won;t hurt trying a few different things to even rule out some type of malware or virus infection. The typical folly of any trojan downloader is creating new entries in the system registry to see more bugs brought onto the system.