Slowness is normally an issue best solved by resetting your MiFi. However, you mention that your laptop does not have slowness issues with the MiFi which leads me to think that the problem is more isolated to something on your desktop. Your desktop may have accumulated viruses and clutter over the years which have slowly deteriorated its performance. I suggest starting with a reload of your desktop from the CD's and starting fresh.
If you do not wish to go down that path then you can try some lowering your wireless mode to help with the disconnections:
Change the 802.11/Wireless Mode
- Sign into your MiFi
- Click the Wi-Fi tab
- Change the 802.11 mode from B+G+N to B+G
- Click Apply
- Test your connection