1. Processor is a pentium 3, it's pretty slow and you should get that upgraded if you're doing more intensive stuff on pc
2. did you defragment your hard drive?
3. you should have at minimum 15% of free space on hard drive
4. do check for malware, as malware compromising computer can cause slowness
5. Firefox is not really slower than explorer but be sure you limit how many of those addons you got to reduce memory usage. I have firefox and it runs fine on my pc/
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But if you can save money up for a new PC, newegg.com has good deals now and then to check out.